Update docs

[yasahi-hpc]

This PR aims at final updating of docs before release.

1. files under docs updated based on recent changes
2. README.md updated
3. fix issues in Doxygen

[pzehner]

This is a nice update, but I think that information in the documentation shouldn't be duplicated in the readme.

[pzehner]

I think it's not the best idea to duplicate install instructions on both the readme and on the doc. This is an error-prone process (DRY). Since GitHub can display RST files, a different approach is possible.

You should have a condensed section on how to use KokkosFFT as a subdirectory dependency (this is likely how a programmer would start using this library), then have an anchor to the documentation where the complete install process is given:

Further installation details are provided in the [documentation](./docs/intro/building.rst).

I think this approach could be used for other sections of the readme. In other words, this file (which is the first contact for a potential user) would be a simple starter, redirecting to more complete documentation pages.

[yasahi-hpc]

It is indeed a duplicate if people read both README.md and docs. However, people may not necessarily read the docs. Not sure what is the right way.

[yasahi-hpc]

Close this PR, because of asyucrhouns commits issue.