Docs helpers need to manage the future release placeholder %%NEXT%%

[mmattel]

Referencing: #9312 (docs: use placeholder for next release)

With the introduction of %%NEXT%% as placeholder in new variables for upcoming releases (the release number is not known at the time), we need to adapt the docs helper process to recognize that placeholder and replace it in the outcoming adoc (md) files with next (suggestion). Else the placeholder will be printed literally.

As we use next in our admin docs in the URL for the release in progress, using next as name for the introduction version sounds valid to me. Short, precise and in-line. Other suggestions welcomed.

NOTES:

• Just checked, we currently do not use in the admin docs the introduction version EXCEPT for deprecations where the release placeholder is no issue as the release is defined.
• We cant use antora attributes to query the content from the antora side. There are some rules we do not satisfy. This means we must do any replacement in the helpers code if we are going to use the introduction version.
• With the amount of envvars and it is quite hard to detect which ones have been added in which version, I am in favor to extend the output of the adoc table by the introduction version and get that printed in the table. Doing so would need a small change in the helper and the template. Nicely, no backport would be necessary.

@micbar @dragonchaser

[dragonchaser]

This is a minor change to the generator here: https://github.com/owncloud/ocis/blob/master/docs/helpers/templates/adoc-generator.go.tmpl. Also needs adaption of the adoc templates. Since the introductionVersion is not rendered atm I see this as low prio.