Untrack geany.html and automatically enable documentation bulding

As it is generated by the build system it doesn't have to be checked in, and having it in became a whole lot more annoying since it started being generated automatically on `make all` instead of specific (and weird) `make doc` (86cd6c50274ef02da3e6abfb4957e9483b8044a8), as it means whenever the documentation changes the HTML is re-generated on any make run.  This is particularly problematic if using a different Docutils version than the one used to generate the checked-in version as it would create local noise that should not be committed, yet keep being annoying.

This situation probably leads to most people disabling (or at least, not enabling) the documentation generation on normal builds, itself leading to more hassle updating of the documentation.

As the generated HTML would not be annoying anymore, we can enable automatic enabling of HTML and PDF documentation generation when appropriate Docutils tools are found, as it doesn't create noise any more.

Finally, this is not a problem for generating tarballs on `make dist` as it specifically forces enabling of all three documentation targets (API, HTML and PDF).