Add new model docs (#9667)

* add new model logic

* fix docs

* change structure

* improve add_new_model

* push new changes

* up

* up

* correct spelling

* improve docstring

* correct line length

* update readme

* correct links

* correct typos

* only add rst file for now

* Apply suggestions from code review 1

Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>
Co-authored-by: Bram Vanroy <Bram.Vanroy@UGent.be>

* Apply suggestions from code review

Co-authored-by: Bram Vanroy <Bram.Vanroy@UGent.be>
Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>
Co-authored-by: Stefan Schweter <stefan@schweter.it>
Co-authored-by: Bram Vanroy <Bram.Vanroy@UGent.be>

* Apply suggestions from code review

Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>
Co-authored-by: Pierric Cistac <Pierrci@users.noreply.github.com>

* finish adding all suggestions

* make style

* apply Niels feedback

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* apply sylvains suggestions

Co-authored-by: Stas Bekman <stas00@users.noreply.github.com>
Co-authored-by: Bram Vanroy <Bram.Vanroy@UGent.be>
Co-authored-by: Stefan Schweter <stefan@schweter.it>
Co-authored-by: Pierric Cistac <Pierrci@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

templates/adding_a_new_model/open_model_proposals/README.md

@@ -0,0 +1,3 @@
+Currently the following model proposals are available:
+
+- TODO(PVP) add BigBird