chore: Add support for hierarchical docs

[silvanocerza]

Proposed Changes:

Add parentDocSlug in API reference docs frontmatter.
Set renderer.parent_doc_slug in docs config yamls to create the hierarchy.

How did you test it?

I generated docs locally and uploaded them to Readme.io to verify the hierarchy would be created correctly.

Notes for the reviewer

An empty or unset parent_doc_slug won't make render or upload fail.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[coveralls]

Pull Request Test Coverage Report for Build 5476992039

• 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 44.659%

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Totals
Change from base Build 5476747209:	0.0%
Covered Lines:	10281
Relevant Lines:	23021

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💛 - Coveralls