[Mesa-dev] Restructure documents directory layout

[Emil Velikov]

As suggested by Jose, here is a short patchset that handles the
initial stage of beating some structure into the docs directory

* All the release notes (both text and html) have been moved to
relnotes/, with filenames indicating the version only
* Mesa/WL extensions have been moved to specs/, including the
OLD ones. All the extensions are now listed in extensions.html

* All the relevant links have been updated, including the missing
relnotes from the index.
* Finally a small docs update due to the recent
"use PACKAGE_VERSION rather than MESA_VERSION" commits

Jose, everyone

Can you give me some insights what you would expect in
 docs/apps/
 docs/devel/ 

Cheers
Emil

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[PATCH 1/4] docs: restructure release notes into separate folder
[PATCH 2/4] docs: move specs to a separate folder
[PATCH 3/4] docs: Add some missing release notes
[PATCH 4/4] docs: Update 'Making new mesa release'