data, devel-docs, gitlab-ci: improve the docs tarball.

- Use a relative path for GIMP_LOGO_PATH inside the gi-docgen generated
  HTML, and not an absolute path taken from build dir (otherwise this
  would break, for installed docs, but also for the tarball and the
  developer website!).
- Also use either gimp-logo.png or gimp-devel-logo.png depending on
  whether we are on a stable or unstable branch.
- Install these in images/ inside the GIMP docs folder, which
  corresponds to the relative path given to GIMP_LOGO_PATH.
- The installed root dir will be $datadir/doc/gimp-2.99/, e.g.
  /usr/share/doc/gimp-2.99/
- Inside this folder, the library references will be in libgimp-3.0/ and
  libgimpui-3.0/ (instead of weird Gimp-3.0/ and GimpUi-3.0/). Note that
  the root dir uses the application version (2.99) whereas the library
  folder use the API versions. These are different in development phase.
- Archive the gi-docgen installed files, not taken from the build dir,
  to avoid packaging temp files, such as the .toml files. Note that
  `g-ir-docs` files are still taken from the build dir, as we don't
  install them yet.
- Finally package all this in a directory before archiving in a tar.xz,
  named the same as the directory (e.g. gimp-api-docs-2.99.13/ inside
  gimp-api-docs-2.99.13.tar.xz).

devel-docs/reference/gimp-ui/meson.build

@@ -8,13 +8,13 @@ gimp_ui_doc_toml = configure_file(
   output: '@BASENAME@',
   configuration: {
     'GIMP_VERSION': gimp_version,
-    'GIMP_LOGO': gimp_logo_dir / 'gimp-logo.png',
+    'GIMP_LOGO': '../images/' + (stable ? 'gimp-logo.png' : 'gimp-devel-logo.png'),
   },
 )
 
 gimp_ui_docs = custom_target('gimp-ui-docs',
   input: libgimpui_gir[0],
-  output: 'GimpUi-@0@'.format(gimp_api_version),
+  output: 'libgimpui-@0@'.format(gimp_api_version),
   command: [
     gi_docgen,
     'generate',
@@ -34,5 +34,5 @@ gimp_ui_docs = custom_target('gimp-ui-docs',
   ],
   build_by_default: true,
   install: true,
-  install_dir: get_option('datadir') / 'doc',
+  install_dir: get_option('datadir') / 'doc' / 'gimp-@0@'.format(gimp_app_version),
 )