chore(renovate): update GitHub Actions major versions in docs (#7584)

## Summary

Originally wanted to update the reference to `astral-sh/setup-uv` in
https://docs.astral.sh/uv/guides/integration/github/, but thought it
could be nice to automate those updates through Renovate. The custom
manager will look for any major version GitHub Action reference in any
Markdown file in `docs` directory, and raise a PR to update it.

Possible improvements:
- We could separate those updates from updating the actions updates for
uv's own GitHub Actions workflow, which would end up raising 2 different
PRs instead of grouping them (example of the current behaviour without
this improvement in
https://github.com/mkniewallner/mkv-playground/pull/4 where we update
the doc reference at the same time as a real dependency usage in a
workflow).
- ~Should the PRs be raised immediately, to handle the update as soon as
possible, instead of waiting for the regular weekly Monday schedule?
This would ensure that `astral-sh/setup-uv` references are handled as
early as possible.~ done in
6af7f45750

## Test Plan

I've tested that with
00ddfb6900/renovate.json5
and
00ddfb6900/docs/integeration/foo.md,
where Renovate raised 2 PRs:
- https://github.com/mkniewallner/mkv-playground/pull/13
- https://github.com/mkniewallner/mkv-playground/pull/4

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

.github/renovate.json5

@@ -25,6 +25,14 @@
       matchManagers: ["cargo"],
       enabled: false,
     },
+    {
+      // Create dedicated branches to update references to dependencies in the documentation.
+      matchPaths: ["docs/**/*.md"],
+      commitMessageTopic: "documentation references to {{{depName}}}",
+      semanticCommitType: "docs",
+      semanticCommitScope: null,
+      additionalBranchPrefix: "docs-",
+    },
     {
       // Group upload/download artifact updates, the versions are dependent
       groupName: "Artifact GitHub Actions dependencies",
@@ -64,6 +72,18 @@
       enabled: false,
     },
   ],
+  customManagers: [
+    // Update major GitHub actions references in documentation.
+    {
+      customType: "regex",
+      fileMatch: ["^docs/.*\\.md$"],
+      matchStrings: [
+        "\\suses: (?<depName>[\\w-]+/[\\w-]+)(?<path>/.*)?@(?<currentValue>.+?)\\s",
+      ],
+      datasourceTemplate: "github-tags",
+      versioningTemplate: "regex:^v(?<major>\\d+)$",
+    },
+  ],
   vulnerabilityAlerts: {
     commitMessageSuffix: "",
     labels: ["internal", "security"],