convert doc/Makefile to CMake

* CMake will build the pdf version of the manual if `texi2pdf` is installed.
It will be installed to `DOCDIR/ledger{,3}.pdf`, for example
`/usr/local/share/ledger/ledger3.pdf`.
Also, the man page will be installed to `MANDIR/man1/ledger.1`.
* The option `BUILD_DOCS` is now on by default.
* A new option `BUILD_WEB_DOCS` is used to toggle the generation
of the html version of the manual and the man page (off by default).
* All this is added to the `doc` make target.

Note that the `man2html` rule is untested as I don't have that program.