DOCS: add documentation and example for sphinx-proof (#1499)

* DOCS: add documentation on sphinx-proof for proof, algorithm style admonitions

* Update docs/content/proof.md

Co-authored-by: Chris Holdgraf <choldgraf@berkeley.edu>

* Update docs/content/proof.md

Co-authored-by: Chris Holdgraf <choldgraf@berkeley.edu>

* Update docs/content/proof.md

Co-authored-by: Chris Holdgraf <choldgraf@berkeley.edu>

* Update docs/content/proof.md

Co-authored-by: Chris Holdgraf <choldgraf@berkeley.edu>

* edits and incorporate feedback from @choldgraf

Co-authored-by: AakashGC <aakashguptachoudhury@gmail.com>
Co-authored-by: Chris Holdgraf <choldgraf@berkeley.edu>