[gi-docgen] (7 commits) ...Merge branch 'wip/carlosg/multi-content-dir' into 'main'

[Emmanuele Bassi <ebassi src gnome org>]

Summary of changes:

  b390c78... utils: Add helper method to lookup an extra content file (*)
  839a0a0... genindices: Handle multiple --content-dir arguments (*)
  8e115f1... gendeps: Handle multiple --content-dir arguments (*)
  9bd1ea1... generate: Handle multiple --content-dir arguments (*)
  b58e939... docs: Document --content-dir as a multi argument (*)
  e3a6d47... utils: Raise exception for missing content files (*)
  6485146... Merge branch 'wip/carlosg/multi-content-dir' into 'main'

(*) This commit already existed in another branch; no separate mail sent