feat: Stage 2 of nox implementation - adding docs target

[mf2199]

As suggested in #466 , this represents the second item of the recommended list aimed to implement nox testing automation in multiple steps.

Change list:

1. The docs target added to Nox configuration;
2. New Sphinx configuration file;
3. Default HTML and style sheet templates;
4. Initial versions of readme and changelog files;
5. Sample reST doc stubs, later to be expanded with more actual documentation;
6. Necessary updates to the .gitignore.

Towards #474.

[mf2199]

Docs session test log:

nox > Running session docs
nox > Creating virtual environment (virtualenv) using python.exe in .nox\docs
nox > pip install -e .
nox > pip install sphinx<3.0.0 alabaster recommonmark
nox > sphinx-build -W -T -N -b html -d docs\_build\doctrees\ docs\ docs\_build\html\
Running Sphinx v2.4.4
making output directory... done
[autosummary] generating autosummary for: README.rst, api-reference.rst, changelog.md, connection-usage.rst, index.rst
loading intersphinx inventory from http://python.readthedocs.org/en/latest/objects.inv...
loading intersphinx inventory from https://google-auth.readthedocs.io/en/stable/objects.inv...
loading intersphinx inventory from https://googleapis.dev/python/google-api-core/latest/objects.inv...
loading intersphinx inventory from https://grpc.io/grpc/python/objects.inv...
intersphinx inventory has moved: http://python.readthedocs.org/en/latest/objects.inv -> https://python.readthedocs.io/en/latest/objects.inv
intersphinx inventory has moved: https://grpc.io/grpc/python/objects.inv -> https://grpc.github.io/grpc/python/objects.inv
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 5 source files that are out of date
updating environment: [new config] 5 added, 0 changed, 0 removed
reading sources... [ 20%] README
reading sources... [ 40%] api-reference
reading sources... [ 60%] changelog
c:\git\python-spanner-django\.nox\docs\lib\site-packages\recommonmark\parser.py:75: UserWarning: Container node skipped: type=document
  warn("Container node skipped: type={0}".format(mdnode.t))
reading sources... [ 80%] connection-usage
reading sources... [100%] index

looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [ 20%] README
writing output... [ 40%] api-reference
writing output... [ 60%] changelog
writing output... [ 80%] connection-usage
writing output... [100%] index

generating indices...  genindexdone
writing additional pages...  searchdone
copying static files... ... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded.

The HTML pages are in docs\_build\html.
nox > Session docs was successful.

[mf2199]

The .kokoro error points to the same parallelize_tests.go:

panic: rpc error: code = ResourceExhausted desc = Project 1065521786570 cannot add 1 nodes in region us-west1.

goroutine 1 [running]:
main.main()
	/Users/emmanuelodeke/Desktop/django-spanner/parallelize_tests.go:81 +0xfa1

[ID: 9463495] Build finished after 191 secs, exit value: 2

I'd suggest to remove or at least disable this parallelization for now, as it's benefit/trouble ratio seems to be less than unity.

[c24t]

I'd suggest to remove or at least disable this parallelization for now

As @IlyaFaer points out in #471 (comment), we probably can't outright remove the parallelization without kokoro timing out. But hopefully we can split it into separate parallel kokoro checks.

[c24t]

A few comments, the only substantial one is about the ../README.md line.