Reorganize API docs into subpages (backport #927)

[mergify]

This is an automatic backport of pull request #927 done by Mergify.
Cherry-pick of bca9271 has failed:

On branch mergify/bp/stable/0.13/pr-927
Your branch is up to date with 'origin/stable/0.13'.

You are currently cherry-picking commit bca9271.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   docs/source/api/algorithm_functions/centrality.rst
	new file:   docs/source/api/algorithm_functions/connectivity_and_cycles.rst
	new file:   docs/source/api/algorithm_functions/dag_algorithms.rst
	new file:   docs/source/api/algorithm_functions/graph_operations.rst
	new file:   docs/source/api/algorithm_functions/index.rst
	new file:   docs/source/api/algorithm_functions/isomorphism.rst
	new file:   docs/source/api/algorithm_functions/link_analysis.rst
	new file:   docs/source/api/algorithm_functions/matching.rst
	new file:   docs/source/api/algorithm_functions/other.rst
	new file:   docs/source/api/algorithm_functions/shortest_paths.rst
	new file:   docs/source/api/algorithm_functions/traversal.rst
	new file:   docs/source/api/algorithm_functions/tree.rst
	new file:   docs/source/api/converters.rst
	new file:   docs/source/api/custom_return_types.rst
	new file:   docs/source/api/exceptions.rst
	new file:   docs/source/api/generators.rst
	new file:   docs/source/api/graph_classes.rst
	new file:   docs/source/api/index.rst
	new file:   docs/source/api/layout_functions.rst
	new file:   docs/source/api/pydigraph_api_functions.rst
	new file:   docs/source/api/pygraph_api_functions.rst
	new file:   docs/source/api/random_graph_generator_functions.rst
	new file:   docs/source/api/serialization.rst
	modified:   docs/source/index.rst
	modified:   docs/source/install.rst

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: docs/source/api.rst

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[coveralls]

Pull Request Test Coverage Report for Build 5649602085

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.383%

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Totals
Change from base Build 5605320059:	0.0%
Covered Lines:	15004
Relevant Lines:	15567

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💛 - Coveralls

[Eric-Arellano]

Replaced by #940