[docs] Reorganise API Reference

[akihironitta]

What does this PR do?

Description of the changes (TODO)

• Move tutorials from "API Reference" to "Guides"
• Include all public APIs (under docs/build/api/generated/)
  • flash
  • flash.core.serve
  • other minor classes/functions
• Create API tables like PL's one or PyTorch's

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• Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes?
• [n/a] Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• [n/a] If you made a notable change (that affects users), did you update the CHANGELOG?

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[codecov]

Codecov Report

Merging #555 (20efd53) into master (78867ad) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
- Coverage   91.40%   91.40%   -0.01%     
==========================================
  Files         117      117              
  Lines        7191     7190       -1     
==========================================
- Hits         6573     6572       -1     
  Misses        618      618              

Flag	Coverage Δ
unittests	91.40% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/data/callback.py	100.00% <ø> (ø)
flash/core/registry.py	94.82% <ø> (ø)
flash/core/trainer.py	84.61% <ø> (ø)
flash/core/utilities/imports.py	89.53% <ø> (-0.13%)	⬇️
flash/text/seq2seq/question_answering/model.py	76.47% <ø> (ø)
flash/core/finetuning.py	88.88% <100.00%> (ø)

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[ethanwharris]

@akihironitta This is awesome! Does it still need to be a draft?

[akihironitta]

@ethanwharris Yes, sorry for the delay. I'll try to finish this PR within an hour.

[ethanwharris]

@akihironitta No rush, I'm just excited to get it in 😃 Looks great!

[ethanwharris]

This is awesome! Just a minor comment