Use substitutions within an include

[stevepiercy]

Links of affected pages in Plone Documentation, if any.

• https://6.docs.plone.org/install/install-from-packages.html#nvm
• https://6.docs.plone.org/volto/contributing/developing-core.html#nvm

Description

Both of the above installation instructions could be replaced with an include, but the NVM_VERSION substitution works only on the Sphinx source-read event until we upgrade to Sphinx 7.2.5 to be able to use the include-read event.

We want to move to includes wherever possible to avoid copy-pasting of documentation, especially for installation docs.

See also plone/volto#3289 and plone/volto#5564.