Add external documentation for MDP

[rod-glover]

• [x] TODO: Rebase this on master after PR #269 is merged.

Important

Do NOT merge this branch until:

1. PR #269 is merged
2. MDP is deployed to production

Changes

After merging PR #269 , the PDP will be able to download user documentation for external portals from GitHub and integrate it into the PDP user documentation suite.

Use that capability to add user documentation for the MDP portal.

Resolves #271

Checklist

• [x] Full integration tests are passing locally
• [x] Docs have been updated to reflect the changes

[rod-glover]

Checked that docs are created correctly in local deployment