Also include the documentation MarkDown pages in the Doxygen docs.
These will show up as Related Pages in the interface.
* The formatting of `INSTALL-*.md` had to be changed some:
- Doxygen doesn't support headings in bulleted lists (lines starting
with `* ###`) -- I'm not even sure that's legal markdown. They were
changed to just level 3 headings (lines starting with `###`).
- ALL Windows paths in `INSTALL-WINDOWS.md` were wrapped in backticks,
to prevent Doxygen parsing them as markup commands.
- Level 1 headings were added to the top of the three install docs,
giving them the title "Building libopenshot for ___(OS)___".
Otherwise all three pages were titled "Getting Started".
* Separately, the table at the top of `HW-ACCEL.md` does not
translate well to Doxygen. It will need further polishing. But the
docs are all quite readable now.
Doxygen was having real trouble generating clean docs from
the JUCE sources, but their own website docs were generated
with Doxygen as well, and looked great.
Turns out, they preprocess their own source files before
generating docs from them. Borrowed their Python script to
munge the sources, and connected it up as a new target
dependency for `make doc`. It'll only run if the docs are
requested to be built, and `make clean` will clear out
the preprocessed sources if it's run.
