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This makes all common files "port-aware" using the .. only directive.

Using Damien's approach where conf.py and topindex.html are
shared by all ports.

Should have done it before tagging...

ReadTheDocs needs the root directory in its search path so it finds
topindex.html.

The sphinx_rtd_theme is used by ReadTheDocs to render a pretty looking
documentation.  If you have this theme installed locally then your
locally-compiled docs will look exactly like the published
documentation.   Otherwise it falls back to the default theme.

This needs to be done before actually tagging the new version.

The inline docs (prefixed with /// in .c files) have been converted to
RST format and put in the docs subdirectory.