docs: move ARB_color_buffer_float.txt into specs-folder

Closed Erik Faye-Lund requested to merge kusma/mesa:docs-move-spec into master

All other extension specs are located in docs/specs rather than in the docs root. Looks like this source file was simply placed incorrectly.

So let's move it so it can be closer to its frends.

Fixes: e5c6a92a ("mesa: implement clamping controls (ARB_color_buffer_float)")

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