CONTRIBUTING.md: sections, mkdocs serve, container rebuild

[LinasKo]

Description

CONTRIBUTING.md now has sections.

Added instructions:

• How to rebuild the dev container
• How to serve the docs (unsure if there's a better way)

Opening a draft PR - let me know if there's a better way to install the dependencies for docs.

Type of change

• [x] This change requires a documentation update

How has this change been tested, please provide a testcase or example of how you tested the change?

1. I rebuilt the container, ran the doc dependency install command I added, served the docs.
2. I checked that contributing.md looks okay in the repo.

Any specific deployment considerations

Is this the best way of installing mkdocs dependencies?

Docs

• [x] Docs updated? What were the changes:

CONTRIBUTING.md:

• How to rebuild the dev container
• How to serve the docs (unsure if there's a better way)