[docs] Version policy needs update for Plone 6.1, Volto 18, Node.js compat

[stevepiercy]

In https://6.docs.plone.org/volto/contributing/version-policy.html, we need to update things for Plone 6.1, Volto 18, and Node.js compatibility.

• Plone, Python, and Plone REST API compatibility needs to insert Plone 6.1, Python 3.10-3.12 (maybe 3.13 upon final release), and Volto 18.
• Set upper bounds of Node.js support for each version of Volto, starting with Volto 17. This information might be better represented in tabular format.
• Replace Supported browsers with the include from Supported web browsers.
• I'm not sure whether we should mention cookiecutter support on this page, both cookiecutter-plone-starter (deprecated) and Cookieplone.

Can I get verification of the above points from the @plone/volto-team? I can do the work.

See:

• Unexpected non-whitespace character after JSON in @plone/volto #6528
• Update docs for Plone 6.1 release documentation#1721
• PLIP: drop Python 3.8 and 3.9 in Plone 6.1 Products.CMFPlone#3926

[sneridagh]

@stevepiercy #6541

[davisagli]

Fixed by #6541 -- I think