Add lint rule that headings must increment one level at a time

Also fixes instances in Markdown that broke the rule.

4 files changed, 16 insertions, 16 deletions

diff --git a/doc/development/changelog.md b/doc/development/changelog.md
index f13c447fef2..3ed586f07e9 100644
--- a/doc/development/changelog.md
+++ b/doc/development/changelog.md
@@ -144,7 +144,7 @@ If you're working on the GitLab EE repository, the entry will be added to
 | [`--type`](#--type-or--t)          | `-t`      | The category of the change, valid options are: `added`, `fixed`, `changed`, `deprecated`, `removed`, `security`, `performance`, `other` |
 | `--help`          | `-h`      | Print help message                                                                                                                      |
 
-##### `--amend`
+#### `--amend`
 
 You can pass the **`--amend`** argument to automatically stage the generated
 file and amend it to the previous commit.
@@ -166,7 +166,7 @@ author:
 type:
 ```
 
-##### `--force` or `-f`
+#### `--force` or `-f`
 
 Use **`--force`** or **`-f`** to overwrite an existing changelog entry if it
 already exists.
@@ -184,7 +184,7 @@ author:
 type:
 ```
 
-##### `--merge-request` or `-m`
+#### `--merge-request` or `-m`
 
 Use the **`--merge-request`** or **`-m`** argument to provide the
 `merge_request` value:
@@ -199,7 +199,7 @@ author:
 type:
 ```
 
-##### `--dry-run` or `-n`
+#### `--dry-run` or `-n`
 
 Use the **`--dry-run`** or **`-n`** argument to prevent actually writing or
 committing anything:
@@ -216,7 +216,7 @@ type:
 $ ls changelogs/unreleased/
 ```
 
-##### `--git-username` or `-u`
+#### `--git-username` or `-u`
 
 Use the **`--git-username`** or **`-u`** argument to automatically fill in the
 `author` value with your configured Git `user.name` value:
@@ -234,7 +234,7 @@ author: Jane Doe
 type:
 ```
 
-##### `--type` or `-t`
+#### `--type` or `-t`
 
 Use the **`--type`** or **`-t`** argument to provide the `type` value:
 
diff --git a/doc/development/ee_features.md b/doc/development/ee_features.md
index 6f4a36d4066..dc17b59a9a0 100644
--- a/doc/development/ee_features.md
+++ b/doc/development/ee_features.md
@@ -446,7 +446,6 @@ The disadvantage of this:
   port `render_if_exists` to CE.
 - If we have typos in the partial name, it would be silently ignored.
 
-
 ##### Caveats
 
 The `render_if_exists` view path argument must be relative to `app/views/` and `ee/app/views`.
@@ -973,7 +972,7 @@ For regular JS files, the approach is similar.
    1. An EE file should be created with the EE only code, and it should extend the CE counterpart.
    1. For code inside functions that can't be extended, the code should be moved into a new file and we should use `ee_else_ce` helper:
 
-##### Example:
+#### Example:
 
 ```javascript
   import eeCode from 'ee_else_ce/ee_code';
@@ -1000,7 +999,7 @@ styles are usually kept in stylesheet that is common for both CE and EE, and it
 to isolate such ruleset from rest of CE rules (along with adding comment describing the same)
 to avoid conflicts during CE to EE merge.
 
-#### Bad
+### Bad
 
 ```scss
 .section-body {
@@ -1016,7 +1015,7 @@ to avoid conflicts during CE to EE merge.
 }
 ```
 
-#### Good
+### Good
 
 ```scss
 .section-body {
@@ -1034,7 +1033,7 @@ to avoid conflicts during CE to EE merge.
 // EE-specific end
 ```
 
-### Backporting changes from EE to CE
+## Backporting changes from EE to CE
 
 Until the work completed to merge the ce and ee codebases, which is tracked on [epic &802](https://gitlab.com/groups/gitlab-org/-/epics/802), there exists times in which some changes for EE require specific changes to the CE
 code base.  Examples of backports include the following:
diff --git a/doc/development/fe_guide/security.md b/doc/development/fe_guide/security.md
index 83bb449e54d..47ac87fc895 100644
--- a/doc/development/fe_guide/security.md
+++ b/doc/development/fe_guide/security.md
@@ -1,5 +1,6 @@
 # Security
-### Resources
+
+## Resources
 
 [Mozilla’s HTTP Observatory CLI][observatory-cli] and the
 [Qualys SSL Labs Server Test][qualys-ssl] are good resources for finding
@@ -56,7 +57,7 @@ Some resources on implementing Subresource Integrity:
 
 -->
 
-### Including external resources
+## Including external resources
 
 External fonts, CSS, and JavaScript should never be used with the exception of
 Google Analytics and Piwik - and only when the instance has enabled it. Assets
@@ -64,7 +65,7 @@ should always be hosted and served locally from the GitLab instance. Embedded
 resources via `iframes` should never be used except in certain circumstances
 such as with ReCaptcha, which cannot be used without an `iframe`.
 
-### Avoiding inline scripts and styles
+## Avoiding inline scripts and styles
 
 In order to protect users from [XSS vulnerabilities][xss], we will disable
 inline scripts in the future using Content Security Policy.
diff --git a/doc/development/testing_guide/ci.md b/doc/development/testing_guide/ci.md
index 7a7fca46534..87d48726268 100644
--- a/doc/development/testing_guide/ci.md
+++ b/doc/development/testing_guide/ci.md
@@ -1,6 +1,6 @@
 # GitLab tests in the Continuous Integration (CI) context
 
-### Test suite parallelization on the CI
+## Test suite parallelization on the CI
 
 Our current CI parallelization setup is as follows:
 
@@ -26,7 +26,7 @@ Our current CI parallelization setup is as follows:
 After that, the next pipeline will use the up-to-date
 `knapsack/${CI_PROJECT_NAME}/rspec_report-master.json` file.
 
-### Monitoring
+## Monitoring
 
 The GitLab test suite is [monitored] for the `master` branch, and any branch
 that includes `rspec-profile` in their name.