Add sphinx docs

[c24t]

This PR adds sphinx boilerplate for generating API docs. Most files are generated with sphinx-quickstart and sphinx-apidoc with few changes.

This branch builds on #11, I'll rebase as it's updated.

You can build the html docs from this branch as:

pip install sphinx sphinx_autodoc_typehints sphinx_rtd_theme
sphinx-quickstart
sphinx-apidoc --force -e -o docs opentelemetry
(cd docs; make html)

I also pushed the generated html docs to the sphinx-docs-built branch, which you can check out to view the docs without building them yourself.

[Oberon00]

Apart from the comment LGTM, but of course this can only be merged once the dependent #11 is merged.

[Oberon00]

Since these options seem to be the same everywhere, they should be added to the configuration https://www.sphinx-doc.org/en/master/usage/extensions/autodoc.html#confval-autodoc_default_options

[c24t]

Should we (1) document the constructor args in the class docstring as opencensus does or (2) whitelist __init__ and keep the docs there?

Option 1:

and option 2:

I have a slight preference for option 1, but option 2 is more explicit.

[c24t]

I updated the sphinx-docs-built branch, see the generated html docs there.

[c24t]

I update the docs for the package move in #21 and removed a lot of the generated API doc boilerplate. After the package move autodoc generates empty docs for the new empty packages from #19, I removed these by hand.

I also moved the opentelemetry.trace module docs up to the top-level TOC with the expectation that we'll curate the TOC ourselves instead of leaving it up to autodoc. These changes mean we can't just run sphinx-apidoc and make html to generate the docs; we have to prune some generated files by hand.