[DOC] Organization of documentation

[duydl]

• Fix some undefined page in developer
• Remove installation from navbar. It is a single page, and already linked in index, getting started and documention. The page itself is a section of documentation.

Discussion on organizing get involved, developer, and about (they referencing each other section) // making user guide its own location (like in docs numpy and scikitlearn) though for now there are not enough tutorial there.

Does your contribution introduce a new dependency? If yes, which one?

What should a reviewer concentrate their feedback on?

Did you add any tests for the change?

Any other comments?

PR checklist

For all contributions

• I've added myself to the list of contributors with any new badges I've earned :-)
  How to: add yourself to the all-contributors file in the sktime root directory (not the CONTRIBUTORS.md). Common badges: code - fixing a bug, or adding code logic. doc - writing or improving documentation or docstrings. bug - reporting or diagnosing a bug (get this plus code if you also fixed the bug in the PR).maintenance - CI, test framework, release.
  See here for full badge reference
• Optionally, for added estimators: I've added myself and possibly to the maintainers tag - do this if you want to become the owner or maintainer of an estimator you added.
  See here for further details on the algorithm maintainer role.
• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.

For new estimators

• I've added the estimator to the API reference - in docs/source/api_reference/taskname.rst, follow the pattern.
• I've added one or more illustrative usage examples to the docstring, in a pydocstyle compliant Examples section.
• If the estimator relies on a soft dependency, I've set the python_dependencies tag and ensured
  dependency isolation, see the estimator dependencies guide.

[fkiraly]

Thanks a lot!
Yes, important points to discus.

Remove installation from navbar. It is a single page, and already linked in index, getting started and documention. The page itself is a section of documentation.

I disagree with this removal, installation is one of the "frequent requests" a reader has, so I think it is well justified to have it as one of the quicklinks at the top.