docs: drop stray terraformvyos.rst and remove .rst from source_suffix

The .md equivalent already exists alongside it; with both present and
`source_suffix = ['.rst', '.md']`, Sphinx silently preferred the .rst.
Deletion removes the shadowing and the only active .rst left in tree
(legacy is archived under docs/_rst_legacy/).

Restricting source_suffix to ['.md'] prevents future stray .rst files
from being picked up, and is what external indexers (e.g. context7)
look at when classifying the source format of the docs.

🤖 Generated by [robots](https://vyos.io)

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