Finish codejail-service docs

[timmc-edx]

Fill out remaining documentation under docs/ and in README.rst.

Acceptance Criteria

• First pass: docs: First pass at docs, mostly around config and deployment #25
• More details:
  • Recommendation to harvest apparmor denial logs, e.g. SYSTEMD_COLORS=false journalctl -k --grep='apparmor.*openedx_codejail_service' -f
• Move developer docs from README into new docs/developing.rst page
• Add ADR with repo purpose
• Add ADR explaining the lack of authentication and lack of a DB (summary: allows locking down networking)

PRs:

• docs: Filling in remaining docs #57

[timmc-edx]

Moved Swagger and RTD portions to #53