Document help wanted issue creation process

Contributions without tickets, including raising a proposal for a help wanted issue, are documented here

https://developers.mattermost.com/contribute/getting-started/contributions-without-ticket/

However, core committer process for creating these Help Wanted issues is not documented. Propose we do so. The process is roughly:

1. Create a Jira issue with a clear title and description, including any information that would help a contributor get started.
2. If applicable, have the issue reviewed by a PM or another core committer
3. Add "Help Wanted" fix version, which automatically creates a Help Wanted GitHub issue for that Jira issue. This automation is done via the Zapier tool.
4. Follow up on questions from contributors on the help wanted issue

Note: As of August 6, the Zapier automation is broken due to an issue on Zapier's end. Hence, you will need to create the issue manually on step 3. For an example issue with an intro that helps new contributors to get started can be found here: https://github.com/mattermost/mattermost-server/issues/11675

Start with the contributions without a ticket and core committers pages linked in the issue, then review example GitHub issue #11675. Document the core committer process for creating Help Wanted issues, including Jira preparation, review, the Help Wanted fix version or manual creation while Zapier is broken, and contributor follow-up; done means the process is clear and current.