binman: Add Sphinx extension to auto-generate entry and bintool docs

binman: Add Sphinx extension to auto-generate entry and bintool docs

Currently entries.rst and bintools.rst are generated manually by running
'binman entry-docs' and 'binman bintool-docs', then committed to the
repo. This means the docs can drift out of date when docstrings are
updated but the RST files are not regenerated.

Add a Sphinx extension (binman_docs) that provides two custom
directives:

    .. binman-entry-docs::
    .. binman-bintool-docs::

These parse the etype and btool source files using the ast module to
extract class docstrings, then insert the documentation directly into
the document tree. This avoids the need to import binman modules (which
have dependencies like libfdt that are not available in the ReadTheDocs
build environment) and avoids writing any intermediate files.

Signed-off-by: Simon Glass <sjg@chromium.org>