Make 2.0.0 release

[kghbln]

Tracking issue for the 2.0.0 release.

• Release notes
• Changes since 1.2.0

• [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 "RELEASE NOTES" (start new section)
  • [x] Update "extension.json" (new alpha version)

[oetterer]

@kghbln rest is up to you, I suppose...

[kghbln]

@oetterer Thanks for your help!