Draft: governance: add a governance, processes, and policies page

This is really WIP, but we had some discussion today in the CC meeting about executing projects (pay for project management, max 25% development costs), and I had the idea to create such a page for a while. In addition to the TODOs in the page, things missing:

• Add it so it's searchable in the website
• Figure out duplication with https://postmarketos.org/team/
• Figure out if we can replace https://wiki.postmarketos.org/wiki/Team_members#Trusted_Contributors
• Remove https://gitlab.postmarketos.org/pmos-admin/infra/-/blob/master/docs/board.md?ref_type=heads though we are still missing where to put the email.
• Add policy for reimbursements, and possibly template: https://gitlab.postmarketos.org/pmos-admin/invoices-template

Some things here might come as a surprise to some, since we haven't communicated that much some of these things (project prioritization, Board). I'm happy to go into details for some of them before we merge this.