Document .no_exist folder

[Wauplin]

Resolve #1011, following #986 that introduced the .no_exist folder.

It has never been properly documented. Also exposes the _CACHED_NO_EXIST object. Even if it's a "private" object, I think it makes sense to expose it. This feature is quite niche even though it can be really useful (at least for transformers).

cc @sgugger

PR documentation:

• New section in the caching guide
• Reference for try_to_load_from_cache (wasn't generated before)

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

Thanks for documenting those!

[codecov]

Codecov Report

Base: 83.67% // Head: 83.67% // No change to project coverage 👍

Coverage data is based on head (1c3beea) compared to base (bdb9d06).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 1c3beea differs from pull request most recent head f43aca6. Consider uploading reports for the commit f43aca6 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1308   +/-   ##
=======================================
  Coverage   83.67%   83.67%           
=======================================
  Files          47       47           
  Lines        4711     4711           
=======================================
  Hits         3942     3942           
  Misses        769      769           

Impacted Files	Coverage Δ
src/huggingface_hub/__init__.py	75.75% <ø> (ø)
src/huggingface_hub/file_download.py	86.84% <100.00%> (ø)

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[Wauplin]

Good catch. Thanks for the quick review @sgugger
I'm merging it.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.