meta: enable opt-in to auto version bump workflow (#4031)

Added `.auto-version-bump` files to all `@backstage/community-plugin-maintainers`
owned workspaces to opt them in to the automatic version bump process.

Also updated the Plugin Maintainer Guide with instructions for opting in
and clarifying that maintainers remain responsible for reviewing, updating,
and merging the resulting PRs.

Signed-off-by: Beth Griggs <bethanyngriggs@gmail.com>

docs/plugin-maintainers-guide.md

@@ -8,6 +8,7 @@
     - [PR Reviews \& Merging](#pr-reviews--merging)
     - [Issue Triage](#issue-triage)
   - [Version Bumping](#version-bumping)
+- [Opt-in to Automatic Version Bump PRs](#opt-in-to-automatic-version-bump-prs)
   - [Maintaining and patching an older release line](#maintaining-and-patching-an-older-release-line)
     - [Patching an older release](#patching-an-older-release)
 
@@ -31,6 +32,16 @@ Plugin owners should triage issues related to their plugin as needed. The `@back
 
 Plugin owners are expected to run the Version Bump script for their workspace. The process follows the guidance outlined in the [Version Bumping Documentation](https://github.com/backstage/community-plugins/blob/main/docs/version-bump.md).
 
+# Opt-in to Automatic Version Bump PRs
+
+Plugin owners can opt in to automatic version bump PRs by creating an empty .auto-version-bump file in the root of their workspace (`workspaces/${WORKSPACE}/.auto-version-bump`). This signals that your plugin should be included in the batch version bump workflow, which is triggered manually by one of the `@backstage/community-plugins-maintainers` .
+
+These automated PRs are intended as a convenience to open the version bump for you. As the plugin maintainer, you would still be required to:
+
+- Review the PR
+- Make any necessary patches to adopt the upgrade
+- Merge the PR once it's ready
+
 ## Maintaining and patching an older release line
 
 It may be necessary to patch a prior release line of a plugin when users depend on an older, but stable version and while a newer, incompatible, major version of the plugin exists. Typically for these older releases, only major bugs and security issues will need to be remediated. Not every plugin will need this workflow.