Add separators between parts of API documentation

[msimberg]

This adds horizontal lines as separators between examples, functions, classes in the API documentation. Especially in cases where e.g. an example, variable, and versionadded directive are grouped together, it can sometimes be hard to see how they are grouped. It's easy to mistake that the example might belong to the next item.

This also moves the async_rw_mutex example up to the top of the section, before the API documentation. This way it serves as an overview before the details of the API documentation. We do the same in the pika/init.hpp section.

[msimberg]

The current documentation looks like this:

With the separator it looks like this:

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.02% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (aa6ef39)	18217	13769	75.58%
Head commit (a13f619)	18217 (+0)	13766 (-3)	75.57% (-0.02%)

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 (#1384)	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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[aurianer]

Nice! It improves readability quite much, thanks!