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/
• 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.
• Mastodon guidelines
• postmarketOS Change Request process!
• Expenditure policy has some TODOs, and should be approved by team before. Should this be a follow-up? Removed and added to the team instead
• Do we have a list of previous Board members? Should this be in another page?

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.

Follow-ups (we should open issues or similar for these):

• Figure out if we can replace https://wiki.postmarketos.org/wiki/Team_members#Trusted_Contributors
• Add policy for reimbursements, and possibly template: https://gitlab.postmarketos.org/pmos-admin/invoices-template
• Merge https://gitlab.postmarketos.org/pmos-admin/infra/-/merge_requests/205
• Add CoC policies or link to CoC from policies section
• Add new project policy approved in team meeting:

Projects are big tasks which are assigned by the Core Contributors to be
executed by a single or multiple people. Projects often come associated with
a budget, be it through grants or through postmarketOS funds. During its
execution:

* Each project will have somebody associated with it doing project management.
* That person will be responsible for usage of funds. There will be monthly
  written or verbal reports to the Core Contributors meetings.
* If the project is financed, be it through grants or through postmarketOS own
  expenses, there should be budget allocated to project management. Project
  management-related expenses in a project are limited to a maximum of 20% of
  the development expenses.

    makes sense!
    do we also want this for grants / projects not financed from our donations?
        yes, because our name and reputation is on it