fix broken links in docs, make docs builds stricter

[jameslamb]

I noticed at https://rapidsai.github.io/legate-boost/README.html that some links to images and example READMEs are broken.

CI logs have some hints about those and other broken links:

/__w/legate-boost/legate-boost/docs/source/README.md:34: WARNING: 'myst' cross-reference target not found: 'examples/probabalistic_regression/README.md' [myst.xref_missing]
/__w/legate-boost/legate-boost/docs/source/README.md:51: WARNING: 'myst' cross-reference target not found: 'examples/batch_training/README.md' [myst.xref_missing]
/__w/legate-boost/legate-boost/legateboost/legateboost.py:docstring of sklearn.utils._metadata_requests._MetadataRequester.get_metadata_routing:3: WARNING: undefined label: 'metadata_routing'
/__w/legate-boost/legate-boost/legateboost/legateboost.py:docstring of sklearn.utils._metadata_requests.RequestMethod.__get__.<locals>.func:3: WARNING: undefined label: 'metadata_routing'

(build link)

This proposes the following to fix them:

• copying the content of examples/ into the docs source directory at build time (e.g. when running make html)
• adding those examples as hidden entries in the table of contents

And other fixes for the docs:

• removing make.bat file (I don't think anyone is building these docs on Windows, right?)
• running sphinx-build --fail-on-warning, so any warnings (like broken links) will result in loud CI failures
• small typo fixes

Notes for Reviewers

How I tested this

conda env create \
    --name legate-boost-dev \
    --file ./conda/environments/all_cuda-118.yaml

source activate legate-boost-dev

# build + install the library
./build.sh

# build the docs
make -C docs clean html

# host the docs
python -m http.server \
    -d ./docs/build/html \
    1456

Clicked around the docs, saw all the images rendering successfully and links working.

[jameslamb]

thanks!