Remove mention of Gitter from contributor docs (#12188)
@ericholscher mentioned that it's not very active, and we can remove the
mention. :)
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:books: Documentation previews :books:
- User's documentation (`docs`):
https://docs--12188.org.readthedocs.build/12188/
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- Developer's documentation (`dev`):
https://dev--12188.org.readthedocs.build/12188/
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Signed-off-by: Pavithra Eswaramoorthy <pavithraes@outlook.com>
