Add "Support" page

Let's consider having a "Support" page

Proposed structure

- Links:
    User manual
    Known issues/limitations
    Open issues on Gitlab
- FAQs:
    Migrate from https://code.briarproject.org/briar/briar/-/wikis/FAQ
    Expand FAQs
- Get in Touch:
    https://chat.briarproject.org/
    IRC
    Email
- Glossary
- Report a bug
- Request a feature / Give feedback

added Document Documentation T: feature labels

mentioned in issue #29

added S: in progress label

mentioned in merge request !51 (closed)

mentioned in merge request !52 (closed)

mentioned in merge request !53 (closed)

mentioned in merge request !54 (closed)

mentioned in issue briar#2385 (closed)