docs: tidy the module index and table of contents

Drop the Tech Preview note from the landing page, and the API changes
page it links to. The page compares 6.5 against the 6.4 Tech Preview
and has headed the index for eight releases; the 6.5 and 6.8 LTS docs
keep it for anyone still porting from 6.4.

Use short labels in the articles list, as the table of contents and the
examples list already do, and list Scene Queries in the table of
contents, which drives the sidebar navigation.

Change-Id: Ief5b92be73173ae95ce6807706c44f3ecca580fe
Reviewed-by: Kristoffer Skau <kristoffer.skau@qt.io>