Version dependent tags

[story645]

Add a configuration setting that only shows x tag on specific build. The two use cases I'm thinking of:

• dev/internal tags -> tags used for flagging examples that may need fixing where it'd be useful to see the tag on dev to easily find it, but we may not want to expose folks to on main b/c there it'd be a distraction
• versioning tags: it'd be nice to tag examples that are showing features new to a version w/ a version tag, and it's the type of thing where we may want to phase out showing after N versions or do something like if tag version == build version-> tag==what's new or make it a different color to otherwise highlight highlight examples of new features in gallery matplotlib/matplotlib#15920

[story645]

Also just realized this can be achieved trivially with an ifconfig so I think gonna try that for the internal tags 😅

[espwangning]

I tried this in my project, and it works fine for displaying tags for each individual file, but the internal tags will still show in the tag overview page.

I guess the configuration setting is still needed when reading tags in each file to prepare the overview page and tag entry mapping?