docs: drop most autoconf references

docs: drop most autoconf references

There's still a few in here, but those docs are already so out of date
that it probably makes more sense to delete them. Such as the GLES
docs which still claim we only support 1.1 and 2.0, with no mention of
3.x at all.

v2: - Add docs for testing back end (Eric Engestrom)
    - Drop more autootols references
    - meson is now required not recommended
    - Add $PWD

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Matt Turner <mattst88@gmail.com>