Add documentation about the review process

[PGijsbers]

Change

• Add documentation about the review process.
• Add support for dark mode to the doc pages.

While we do not expect end-users to directly interact with the REST API in most cases, they should still be able to.
Additionally, it serves as a reference for developers who want to build services that integrate (part of) this process.
Also adds darkmode option (picks up on default) -- turns out the SVG rendering already did this, so otherwise the SVG flowchart was not legible (it had white arrows on a white background).

note: This documentation does not describe any limitations, because limitations are not yet implemented.
We will add the documentation for limitations when they are implemented/enforced.

How to Test

Render the documentation locally. Of particular interest is the new "Using the API > Uploading Metadata" page.

Checklist

• Tests have been added or updated to reflect the changes, or their absence is explicitly explained.
• Documentation has been added or updated to reflect the changes, or their absence is explicitly explained.
• A self-review has been conducted checking:
  • No unintended changes have been committed.
  • The changes in isolation seem reasonable.
  • Anything that may be odd or unintuitive is provided with a GitHub comment explaining it (but consider if this should not be a code comment or in the documentation instead).
• All CI checks pass before pinging a reviewer, or provide an explanation if they do not.

Related Issues

[PGijsbers]

@mrorro are you able/willing to review this or should i look for someone else?

[mrorro]

@mrorro are you able/willing to review this or should i look for someone else?

I'll try to review it by the end of next week, hopefully earlier, but if you need it before please find someone else.