docs: add contributor guide for issue/PR comment commands

[Ma77Ball]

Feature Summary

First-time contributors have no easy way to discover the slash commands
available on our issues and pull requests. The commands are documented
only in comments inside .github/workflows/comment-commands.yml, which
most contributors never read. We should surface a friendly, copy-pasteable
reference so people can self-assign issues, manage review requests, and
link sub-issues without asking a maintainer.

Proposed Solution or Design

👋 Comment commands

You can drive common housekeeping tasks just by leaving a comment. Type the
command on its own line.

On issues

Command	What it does
/take	Assign the issue to yourself (self-claim it)
/untake	Remove yourself as assignee

To find unclaimed work, search is:issue is:open no:assignee — there's no
"triage" label; the search filter is the triage state.

Linking sub-issues

You can link from either end of the parent/child relationship:

Command	Where to run it	What it does
/sub-issue #12 #13	On the parent	Links #12 and #13 as children of this issue
/unsub-issue #12 #13	On the parent	Unlinks those children
/parent-issue #5	On the child	Sets #5 as this issue's parent
/unparent-issue	On the child	Removes this issue's parent (auto-detected)
/unparent-issue #5	On the child	Removes parent #5 explicitly

You can write references as #12 or bare 12. Cross-repo references like
owner/repo#12 aren't supported and are ignored.

On pull requests (author only)

Command	What it does
/request-review @user [@user ...]	Request reviews from those users
/unrequest-review @user [@user ...]	Cancel those review requests

You can mention teams as @org/team, and @copilot works too. Only the PR
author can use these commands.

Note: Commands must match exactly — /take this won't work, only
/take. Bots are ignored, and you can't self-link an issue or set an
issue as its own parent.

Affected Area

Other

[Ma77Ball]

/take