Add ActiveJob to "Welcome to Rails" section

[ci skip]
Link to ActiveModel’s README in "Welcome to Rails"

A link to the README of each module is included in "Welcome to Rails",
only ActiveModel (and ActiveJob, see #17264) are missing.

[ci skip]
Remove broken http://tore.darell.no link

Replace with Twitter account, unless @toretore has a better
suggestion :ear since http://tore.darell.no returns 404.

[ci skip]
specify protocol for external links

to ensure correct parsing result of rdoc
Merge pull request #17270 from neumanrq/fix/external_links_to_json_project

specify protocol for external links to json project
Merge pull request #17266 from claudiob/fix-broken-tore-darell-link

Remove broken http://tore.darell.no link
Merge pull request #17265 from claudiob/add-link-to-active-model-readme

Link to ActiveModel’s README in "Welcome to Rails"
Merge pull request #17264 from claudiob/add-activejob-to-readme

Add ActiveJob to "Welcome to Rails" section