feat: manually implement sphinx inv header parsing

[savente93]

Some work let to do, but given that so many other documentation sites are built with sphinx it is still useful to be able to link them. This will hopefully also help me design a good linking system for snakedown itself. For now all we do is parsing, but I also want to add the ability to link to the objects.inv, download and then use that. Some good error handling will also be necessary.

Creating this draft PR to more easily track the progress, even though the history will need some cleanup.

[codecov]

Codecov Report

Attention: Patch coverage is 89.84375% with 39 lines in your changes missing coverage. Please review.

Project coverage is 96.7%. Comparing base (0cf62c9) to head (3f2eb8e).

Files with missing lines	Patch %	Lines
src/indexing/fetch.rs	58.4%	22 Missing ⚠️
src/parsing/sphinx/types.rs	88.8%	13 Missing ⚠️
src/parsing/sphinx/inv_file.rs	98.2%	3 Missing ⚠️
src/indexing/cache.rs	97.7%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/lib.rs	89.5% <ø> (ø)
src/indexing/cache.rs	97.7% <97.7%> (ø)
src/parsing/sphinx/inv_file.rs	98.2% <98.2%> (ø)
src/parsing/sphinx/types.rs	88.8% <88.8%> (ø)
src/indexing/fetch.rs	58.4% <58.4%> (ø)

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