Fix //docs:update (#876)

Fix //docs:update

Also regenerates docs with the new stardoc version.

Right now the command errors out on fresh clones or after a `bazel
clean`.

    $ bazel run //docs:update
    cp: cannot stat 'bazel-bin/docs/packaging.md_': No such file or directory
    cp: cannot stat 'bazel-bin/docs/pip.md_': No such file or directory
    cp: cannot stat 'bazel-bin/docs/pip_repository.md_': No such file or directory
    cp: cannot stat 'bazel-bin/docs/python.md_': No such file or directory

I submitted bazelbuild/stardoc#139 to fix this. @brandjon pointed out
that this should just work as-is, but doesn't because of
bazelbuild/bazel#15043. Until the bazel bug is addressed, we can make
`//docs:update` work by pulling in the latest stardoc version.

One side effect of this patch is that the generated documentation
itself changed a decent amount.

Now the tool works again without errors even after a fresh clone or a
`bazel clean`

    $ bazel run //docs:update
    'bazel-bin/docs/packaging.md_' -> 'docs/packaging.md'
    'bazel-bin/docs/pip.md_' -> 'docs/pip.md'
    'bazel-bin/docs/pip_repository.md_' -> 'docs/pip_repository.md'
    'bazel-bin/docs/python.md_' -> 'docs/python.md'