[WIP] Restructure user guide with subsections

[RDaxini]

• Closes Restructure the user guide with subsections #2302
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

#2302 for discussion

[RDaxini]

Ready for an initial review

One observed limitation: visible main header section titles is currently limited to five headers, so the last one is now kept under "more"
Can this maximum be changed?
If not, I suggest swapping the order of "whatsnew" and "contributing". Having whatsnew at the end seems more intuitive to me, so I'd like to suggest this anyway even if all sections are listed upfront (no "more" list)