Remove duplicate contributing navigation

[stevepiercy]

• Fixes Contributing to projects has duplicate navigation #1582

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[stevepiercy]

The failing workflow Test is due to a new version of Vale getting released. I have a temporary workaround in another PR by pinning it to 2.30.0, and created an issue to fix it, when someone has some time.

[davisagli]

Possibly another use case for https://documatt.com/sphinx-reredirects/index.html (but I can't tell without trying whether it supports including the redirect pages in tables of contents)

But, LGTM as is too.

[stevepiercy]

We need to keep the milestone files in place to avoid Sphinx warnings, and when built they will get clobbered by sphinx-reredirects. This is OK. When I deleted the milestone files, Sphinx complained, for example:

/Users/stevepiercy/projects/Plone/documentation/docs/contributing/index.md:175: WARNING: toctree contains reference to nonexisting document 'contributing/plone-api'

We need the toctree entry, so we need the milestone files in source. Fixed in 1d80cc0

[stevepiercy]

Requested in Discord of AI Team to add four 301 permanent redirects. Right now they return 200, then redirect, which is not great for SEO.

@polyester @fredvd

rewrite ^/contributing/plone-api.html$ https://6.docs.plone.org/plone.api/contribute/index.html permanent;
rewrite ^/contributing/plone-restapi.html$ https://6.docs.plone.org/plone.restapi/docs/source/contributing/index.html permanent;
rewrite ^/contributing/volto.html$ https://6.docs.plone.org/volto/contributing/index.html permanent;
rewrite ^/install/install-from-packages.html$ https://6.docs.plone.org/install/create-project.html permanent;

[polyester]

done