Improve documentation for Autotools, CMake and Meson elements

I find the documentation for the buildelement plugins a bit lacking. This is an attempt to improve matters.

Most users will be able to use the defaults, or just add more config options so I have added some blurb about that. It may be too much ... I'd happily remove the examples.

Also, we now document the expected uses of conf-local, conf-global and conf-extra which was previously a mystery.

And I added links to the relevant build system websites!