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Producing/updating documentation for a specific 'release/branch' is different depending on whether the version is liked to a tag or branch. (it's not consistent - sorry about that it was like that when I found it).
The RTD uses the branches and tags to link versions of the documentation to moments in time in the source, to update the tags and branches seen by RTD, run a build of master
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The reading of tags is dynamic, so you can remove and re-add a tag and then rebuild that 'version'.
If the versions of documentation are linked to branches (ie Installation Docs) then :
Merging a pull request into that branch will cause the version to be built and updated online automatically
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