* Fix clang-tidy return value reporting

In case clang-tidy is invoked manually, i.e. if run-clang-tidy(.py) is
not found, Meson would not report the return value. This is caused by
ignoring the return value of manual_clangformat() in clangformat()
within mesonbuild/scripts/clangtidy.py.

Even though only more recent-versions of clang-tidy actually report an
non-zero exit code if errors are found, there is no reason Meson
shouldn't simply report any error codes it received from clang-tidy.

Fixes #7948.

* Rename methods in clangtidy.py from clangformat to clangtidy

For some unknown reason, the method names in clangtidy.py are clangformat()
and manual_clangformat(). This is confusing, as clang-format is not
invoked by them, clang-tidy is. Hence rename those from

{manual_}clangformat() → {manual_}clangtidy()

Some license identifiers are ambiguous (e.g. "GPL3"). The SPDX license
identifiers avoid this by providing standardized and unique identifiers
(e.g. "GPL-3.0-only" or "GPL-3.0-or-later" for the previous example).

Because SPDX short-form identifiers are also both human- and
machine-readable we should recommend them in the documentation.

More information (advantages, details, etc.) can be found here:
- https://spdx.dev/resources/use/#identifiers
- https://spdx.dev/ids/

Fix #7905.

Using the std option, so now `rust_std=..` will work. I've chosen to use
"std" even though rust calls these "editions", as meson refers to
language versions as "standards", which makes meson feel more uniform,
and be less surprising.

Fixes: #5100

Apparently anchor links are case-sensitive and needs to be lowercase in hotdoc.
Prior to this change the anchor links were uppercase so the link did not refer to the anchor tag.

Likely that there are more occurrences around the docs than this, have not looked.