Add documentation on customisations

[cc-a]

• Issue: [Link to the issue. Do not use any of the github automation keywords]

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This PR adds a documentation framework (mkdocs) and adds detailed documentation covering the customisations made from base InvenioRDM.

Developer Checklist

Developers should review and confirm each of these items before requesting review

• Code meets acceptance criteria from issue
• Unit tests are written and all pass
• User Test Scripts (if required) are written and have been run through
• Code documentation and related non-code documentation has all been updated

Reviewer Checklist

Reviewers should review and confirm each of these items before approval
If there are multiple reviewers, this section can be duplicated for each reviewer

• Code meets acceptance criteria from issue
• Unit tests are written and all pass
• User Test Scripts (if required) are written and have been run through
• Code documentation and related non-code documentation has all been updated
• Migation has been created and tested

Testing

List user test scripts that need to be run

List any non-unit test scripts that need to be run