chore(renovate): enable regex manager (#7820)

## Summary

Investigated
#7807 (comment), and
the reason why the PR mentioned in the comment did not work in the end
is because we only opt-in for specific managers in Renovate
configuration. By enabling
[regex](https://docs.renovatebot.com/modules/manager/regex/) manager, we
should now get proper updates to documentation references.

## Test Plan

Tested enabling specific managers (including `regex` one) in
mkniewallner/mkv-playground#18, and Renovate was
still able to detect regex dependencies.

.github/renovate.json5

@@ -8,7 +8,7 @@
   semanticCommits: "disabled",
   separateMajorMinor: false,
   prHourlyLimit: 10,
-  enabledManagers: ["github-actions", "pre-commit", "cargo"],
+  enabledManagers: ["github-actions", "pre-commit", "cargo", "regex"],
   cargo: {
     // See https://docs.renovatebot.com/configuration-options/#rangestrategy
     rangeStrategy: "update-lockfile",