Change API header titles and info

[dimitraka]

The API documentation now includes the following information:

1. title of the page includes the functions that are documented in each page
2. the header file (along with its link) needed to be included is listed under the page title

For example, instead of the title:
hpx/parallel/algorithms/adjacent_find.hpp

The new title will be:
hpx::adjacent_find
followed by the include information:
Defined in header hpx/algorithm.hpp

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.02%	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (c0d79c8)	201862	171892	85.15%
Head commit (1442a7c)	190688 (-11174)	162330 (-9562)	85.13% (-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 (#6397)	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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[hkaiser]

@dimitraka could you please resolve the conflicts intorduced by a recent merge to HPX master?

[hkaiser]

Thanks @dimitraka! Much appreciated! LGTM.

[hkaiser]

@dimitraka Would you mind having a look at the clang-format issues reported?