docs: move doc/ to docs/

The docs/ directory is special in GitHub, since it can be used to serve GitHub
Pages from, so there's a benefit to switching to it in order to expose it
directly as a website.

Updated references to it from the documentations themselves, from the
CONTRIBUTING.md file and from Meson build files.

.github/CONTRIBUTING.md

@@ -24,8 +24,8 @@ If you discover a security vulnerability, we'd appreciate a non-public disclosur
 ## Posting Pull Requests
 
 * Make sure to post PRs only relative to a very recent git master.
-* Follow our [Coding Style](https://raw.githubusercontent.com/systemd/systemd/master/doc/CODING_STYLE) when contributing code. This is a requirement for all code we merge.
-* Please make sure to test your change before submitting the PR. See [HACKING](https://raw.githubusercontent.com/systemd/systemd/master/doc/HACKING) for details how to do this.
+* Follow our [Coding Style](https://raw.githubusercontent.com/systemd/systemd/master/docs/CODING_STYLE) when contributing code. This is a requirement for all code we merge.
+* Please make sure to test your change before submitting the PR. See [HACKING](https://raw.githubusercontent.com/systemd/systemd/master/docs/HACKING) for details how to do this.
 * Make sure to run the test suite locally, before posting your PR. We use a CI system, meaning we don't even look at your PR, if the build and tests don't pass.
 * If you need to update the code in an existing PR, force-push into the same branch, overriding old commits with new versions.
 * After you have pushed a new version, add a comment about the new version (no notification is sent just for the commits, so it's easy to miss the update without an explicit comment). If you are a member of the systemd project on GitHub, remove the `reviewed/needs-rework` label.

docs/HACKING

@@ -7,7 +7,7 @@ Request (PR):
         https://github.com/systemd/systemd/pull/new
 
 Please make sure to follow our Coding Style when submitting patches. See
-doc/CODING_STYLE for details. Also have a look at our Contribution Guidelines:
+docs/CODING_STYLE for details. Also have a look at our Contribution Guidelines:
 
         https://github.com/systemd/systemd/blob/master/.github/CONTRIBUTING.md
 
@@ -23,7 +23,7 @@ encouraged.
 Please also have a look at our list of code quality tools we have setup for systemd,
 to ensure our codebase stays in good shape:
 
-        https://github.com/systemd/systemd/blob/master/doc/CODE_QUALITY.md
+        https://github.com/systemd/systemd/blob/master/docs/CODE_QUALITY.md
 
 Please always test your work before submitting a PR. For many of the components
 of systemd testing is straight-forward as you can simply compile systemd and

meson.build

@@ -2749,8 +2749,8 @@ subdir('network')
 subdir('man')
 subdir('shell-completion/bash')
 subdir('shell-completion/zsh')
-subdir('doc/sysvinit')
-subdir('doc/var-log')
+subdir('docs/sysvinit')
+subdir('docs/var-log')
 
 # FIXME: figure out if the warning is true:
 # https://github.com/mesonbuild/meson/wiki/Reference-manual#install_subdir
@@ -2766,13 +2766,13 @@ install_data('LICENSE.GPL2',
              'LICENSE.LGPL2.1',
              'NEWS',
              'README',
-             'doc/CODING_STYLE',
-             'doc/DISTRO_PORTING',
-             'doc/ENVIRONMENT.md',
-             'doc/HACKING',
-             'doc/TRANSIENT-SETTINGS.md',
-             'doc/TRANSLATORS',
-             'doc/UIDS-GIDS.md',
+             'docs/CODING_STYLE',
+             'docs/DISTRO_PORTING',
+             'docs/ENVIRONMENT.md',
+             'docs/HACKING',
+             'docs/TRANSIENT-SETTINGS.md',
+             'docs/TRANSLATORS',
+             'docs/UIDS-GIDS.md',
              'src/libsystemd/sd-bus/GVARIANT-SERIALIZATION',
              install_dir : docdir)