Deploy sphinx-produced docs to RTD

[yarikoptic]

• by default should provide the one for the most recent release, with master to provide the version in master
• should also do preview for the PRs, example e.g. https://github.com/con/open-brain-consent/pulls
• Fix RTD build of sphinx docs in master #717

follow up to #712

[jwodder]

@yarikoptic Do you already have a user account on Readthedocs that I can add as a maintainer? After I add you, you need to set up the webhook, which I apparently don't have permission to do.

[yarikoptic]

• site-readthedocs@onerussian.com AKA yarikoptic
• you are the Admin now for dandi-cli

[jwodder]

@yarikoptic Added you and set up the webhook. #715 just needs to be merged now, and then the site can build.

[yarikoptic]

latest (master) fails to build, details from https://readthedocs.org/projects/dandi/builds/14260074/:

Running Sphinx v4.1.1
loading translations [en]... done
making output directory... done
[autosummary] generating autosummary for: cmdline/delete.rst, cmdline/digest.rst, cmdline/download.rst, cmdline/index.rst, cmdline/ls.rst, cmdline/organize.rst, cmdline/shell-completion.rst, cmdline/upload.rst, cmdline/validate.rst, index.rst, modref/consts.rst, modref/dandiapi.rst, modref/dandiarchive.rst, modref/index.rst, modref/support.digests.rst, modref/utils.rst
/home/docs/checkouts/readthedocs.org/user_builds/dandi/envs/latest/lib/python3.7/importlib/_bootstrap.py:219: RuntimeWarning: numpy.ndarray size changed, may indicate binary incompatibility. Expected 80 from C header, got 88 from PyObject
  return f(*args, **kwds)
WARNING: [autosummary] failed to import 'dandi.tests.fixtures': no module named dandi.tests.fixtures
WARNING: [autosummary] failed to import 'dandi.tests.skip': no module named dandi.tests.skip
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 16 source files that are out of date
updating environment: [new config] 16 added, 0 changed, 0 removed
reading sources... [  6%] cmdline/delete
reading sources... [ 12%] cmdline/digest
reading sources... [ 18%] cmdline/download
reading sources... [ 25%] cmdline/index
reading sources... [ 31%] cmdline/ls
reading sources... [ 37%] cmdline/organize
reading sources... [ 43%] cmdline/shell-completion
reading sources... [ 50%] cmdline/upload
reading sources... [ 56%] cmdline/validate
reading sources... [ 62%] index
reading sources... [ 68%] modref/consts
reading sources... [ 75%] modref/dandiapi
reading sources... [ 81%] modref/dandiarchive
reading sources... [ 87%] modref/index
reading sources... [ 93%] modref/support.digests
reading sources... [100%] modref/utils

/home/docs/checkouts/readthedocs.org/user_builds/dandi/checkouts/latest/docs/source/modref/index.rst.rst:44: WARNING: autosummary: failed to import tests.fixtures
/home/docs/checkouts/readthedocs.org/user_builds/dandi/checkouts/latest/docs/source/modref/index.rst.rst:44: WARNING: autosummary: failed to import tests.skip
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [  6%] cmdline/delete
writing output... [ 12%] cmdline/digest
writing output... [ 18%] cmdline/download
writing output... [ 25%] cmdline/index
writing output... [ 31%] cmdline/ls
writing output... [ 37%] cmdline/organize
writing output... [ 43%] cmdline/shell-completion
writing output... [ 50%] cmdline/upload
writing output... [ 56%] cmdline/validate
writing output... [ 62%] index
writing output... [ 68%] modref/consts
writing output... [ 75%] modref/dandiapi
writing output... [ 81%] modref/dandiarchive
writing output... [ 87%] modref/index
writing output... [ 93%] modref/support.digests
writing output... [100%] modref/utils

generating indices... genindex py-modindex done
writing additional pages... search done
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build finished with problems, 4 warnings.

[yarikoptic]

well, we kinda have it, a dedicated issue on failed builds is re-opened to track that particular aspect