Make 2.1.1 release

[kghbln]

Tracking issue for the 2.1.1 release.

• Release notes
• Changes since 2.1.0

• [x] Finalize RELEASE NOTES
• [x] Update "README.md"
• [x] Update "/docs/"
• [x] Update "composer.json"
• [x] Update version number "extension.json"
• [x] Create tag
• [x] Add release to Wikidata
• [x] Update homepage
• [x] Announce via mail
• [x] Do a release tweet on Twitter
• [x] Run composer update on smw.o
• [x] Update "LocalSettings.php"
• [x] Start development of new version
  • [x] Update "composer.json" (update dev alias)
  • [x] Update "extension.json" (new alpha version)

[kghbln]

Due to https://github.com/SemanticMediaWiki/SemanticCite/issues/70