[TRACKER] Document new features in Godot 3.5

[mhilbrunner]

We now got a shiny 3.5 docs branch to document changes in Godot 3.5.
See the changelogs here (by author) and here (all commits).

Calinou started updating the features page in #5722.

• OccluderShapePolygon: Add OccluderShapePolygon docs #5795
• 3D navigation server: Document 3D navigation server in Godot 3.5 #5800
• Physics Interpolation: Physics interpolation docs #5801
• Material overlays: Document Material Overlay #5826
• Theme type variations: Document type variations and update images #5830
• Label3D and text mesh: Document Label3D and text mesh (3.5) #5847

Feel free to edit / comment.

[skyace65]

Just made a PR for the 3D navigation server.

[skyace65]

Made a PR for the new material overlay property (#5826). Calinou already documented .mo and .woff2 support and it's in the 3.5 branch of the docs.

[skyace65]

Made a PR for theme type variations (#5830). It's for the master branch but should be cherrypickable.

[skyace65]

Made a PR for scene unique nodes (#5831). It's for the master branch but should be cherrypickable.

[skyace65]

Made a PR for Label3D and text mesh (#5847).

[mhilbrunner]

I think most of this is done by now. Thanks to all of you who documented all these shiny new things :)