Expand and add more CUDA/HIP documentation

[msimberg]

Adds documentation for classes that weren't yet documented and updates the existing documentation. Adds a section for pika/cuda.hpp to the API documentation.

Early stages, far from complete, but may require some discussion before continuing.

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
✅ -0.13% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (015302c)	18217	13754	75.50%
Head commit (c0ca2c9)	18217 (+0)	13731 (-23)	75.37% (-0.13%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1309)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[msimberg]

I welcome more feedback on this PR now. I feel like some of the examples I've created are a bit forced/artificial in the name of showing off what's possible with the adaptors, so if you have suggestions on how to improve change them that's particularly welcome. I've also now documented the types of the CUDA/HIP sender adaptors so that we can document the parameters of them. I think it looks decent enough, but interested in hearing feedback from others.