make RFC repo public and announce change

[lsf37]

When all other issues are completed:

• make this repo public under seL4 org
• make any docsite and website changes live
• announce change on dev list and RFC discourse
• update Jira RFC project summary to redirect to RFC repo
• can we make Jira RFC project read-only?

[lsf37]

I haven't made the Jira project fully read-only, but have set it so that you can't create new issues or new comments.

The project description unfortunately is not shown very prominently, so we might get questions from people who are used to going to Jira. Have renamed the project to Archived RFCs to make it more clear that there is something else out there.