REVIEW: 128780

This adds an ecm-developer manual that replaces writing-find-modules.md
(a lot of which was upstreamed to CMake's own documentation). It also
adds introductory text to the ecm-*-modules manuals.

This is deliberately modelled very closely on CMake's documentation
system.  It's a hefty patch, because it involved changing all the
documentation to be in reStructuredText format.  I also cleaned up the
copyright/license statements at the same time.

Note that the find modules contain the full license, due to the fact
that ecm_use_find_module() copies them out of the ECM distribution.