- describe kernel command line override
- add OpenStack Ironic selection instructions
- describe datasource selection
- move datasource creation doc to dev section
- remove generator stage from boot stages
- document how to disable cloud-init in howtos
- fix missing kernel-cmdline.rst link
- update outdated security advice

Document the following commits:

 34e8c914df666c937e48f5d1c3add0bd47e4e7eb
 f146fe71733e72b94fad525b8cc9988b1405e760
 250280ada67995a8449b64027b879d01939d2729
 612b4de892d19333c33276d541fed99fd16d3998
 a60c0845806baff72c74603286d048efbafab664
 d1ffbea556a06105d1ade88b4143ad43f53692c4
 02202954c65a7a1cdb9b28703bd0af01edd0e091

Updating the links to the paths in the codebase for the various 
network configuration examples

Use the module level Log instances instead of passing log instances
to the cc_* handlers

Make the docs look more like the documentation produced on Ubuntu.com.

Make the default font size larger for accessibility.

Code literals are now the same size as regular font for the same reason,
but made slightly bolded to help pick them out from text (which should
help e.g. when commands are included as inline text and not a code block.

Give the sidebar headings a higher weight, for easier navigation.

For accessibility, change the weight and spacing between topics in the
side bars to help separate them and make it easier to tell where one link
ends and the next begins.

Running on OpenStack Ironic was broken in 1efa8a0a0,
which prevented a system configured to run on only
Openstack from actually running this ds. This change
also prevents the kernel commandline definition from
working. This change was required to prevent
unnecessarily probing OpenStack on Ec2, and is
therefore still required. 

This commit reverts an earlier attempt[1][2] to
automatically detect OpenStack, due to regression
it caused. Additionally, this change allows a
system that defines a datasource list containing
only [OpenStack] or [OpenStack, None] to attempt
running on OpenStack, overriding ds_detect(). A
datasource list that defines [OpenStack, None]
still falls back to DataSourceNone if OpenStack
fails to reach the IMDS.

This change also lays groundwork for the following
future work:

1. Add support for other datasources
2. Also override datasource checking when the kernel
   command line defines a datasource. This work needs
   to be done manually to support non-systemd systems.

Besides forcing OpenStack to run when it is the only
datasource in the datasource list, this commit also:

[1] 0220295 (it breaks some use cases)
[2] 29faf66 (no longer used)

LP: #2008727