Merge User Guide and API sections in documentation, limit classes to show in API section

[Mr-Geekman]

Before submitting (must do checklist)

• [x] Did you read the contribution guide?
• [x] Did you update the docs? We use Numpy format for all the methods and classes.
• [ ] Did you write any new necessary tests?
• [x] Did you update the CHANGELOG?

Proposed Changes

Look at #1323.

Closing issues

Closes #1323.

[github-actions[bot]]

🚀 Deployed on https://deploy-preview-1324--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #1324 (5ed4a90) into docs-prototype (6135f66) will decrease coverage by 32.19%. The diff coverage is 47.05%.

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@@                 Coverage Diff                 @@
##           docs-prototype    #1324       +/-   ##
===================================================
- Coverage           89.09%   56.90%   -32.19%     
===================================================
  Files                 204      204               
  Lines               12634    12633        -1     
===================================================
- Hits                11256     7189     -4067     
- Misses               1378     5444     +4066     

Files Changed	Coverage Δ
etna/analysis/decomposition/utils.py	36.36% <ø> (-60.01%)	:arrow_down:
etna/analysis/feature_selection/mrmr_selection.py	26.66% <ø> (-73.34%)	:arrow_down:
etna/analysis/forecast/plots.py	15.78% <ø> (-3.05%)	:arrow_down:
etna/auto/__init__.py	100.00% <ø> (ø)
etna/auto/auto.py	28.31% <ø> (-67.39%)	:arrow_down:
etna/auto/pool/generator.py	100.00% <ø> (ø)
etna/clustering/hierarchical/base.py	44.30% <ø> (-53.17%)	:arrow_down:
etna/commands/__init__.py	100.00% <ø> (ø)
etna/core/__init__.py	100.00% <ø> (ø)
etna/datasets/__init__.py	100.00% <ø> (ø)
... and 16 more

... and 120 files with indirect coverage changes

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[alex-hse-repository]

See comments under the issue (i.e. https://github.com/tinkoff-ai/etna/issues/1323#issuecomment-1649718567)