Fix sphinx warnings and turn warnings into errors

[NicolasHug]

Similar to what was done in torchvision pytorch/vision#3290 and torchaudio pytorch/audio#1247

This PR fixes sphinx warnings as well as some other doc layout issues, and sets the

napoleon_numpy_docstring = False
napoleon_google_docstring = True

for sphinx to issue warnings when the wrong parameter syntax is used.

The warnings are now turned into errors. Note: sphinx will now stop compilation at the first warning/error it sees; if we relied on sphinx>=8, we could use the --keep-going option to let it finish.

[codecov]

Codecov Report

Merging #1163 (3d65ca6) into master (651c1f7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1163   +/-   ##
=======================================
  Coverage   79.63%   79.63%           
=======================================
  Files          47       47           
  Lines        3201     3201           
=======================================
  Hits         2549     2549           
  Misses        652      652           

Impacted Files	Coverage Δ
torchtext/datasets/language_modeling.py	90.38% <ø> (ø)
torchtext/datasets/text_classification.py	32.96% <ø> (ø)
torchtext/experimental/vectors.py	90.58% <ø> (ø)
torchtext/experimental/vocab.py	100.00% <ø> (ø)

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[cpuhrsch]

Haha, jinx #1159 - notice that we can also build docs in parallel using the -j flag. I'll rebase.