Add a note on custom validation contexts.

The documentation on `:on` for validations was inconsistent, and most
only referenced the `:create` and `:update` contexts. I fixed those to
be consistent with the documentation on `AM::Validations.validates`,
which seemed to have the best docs.

[ci skip]
[ci skip] add guide for ActiveModel::SecurePassword
'Reversible' ->active_record_migrations [ci skip]

Reversible method documentation moved from migrations.md  to active_record_migrations.md
'Revert' ->active_record_migrations [ci skip]

Revert method documentation moved from migrations.md  to active_record_migrations.md
[ci skip] Fix some typos, normalize sentence.

Follow up of #16722.

https://github.com/rails/rails/pull/16722

also normalize all instance gained methods’ sentence.
form_helpers: demonstrating there is also a collection_select method in form builder
[ci skip] Fix code snippet display in Action Mailer Basics guide.
Consistently capitalize ETag.
Consistently markup etag options.
Use released rails-dom-testing
Use release web-console
Remove internal options from query string of paths

Fixes #17057
Improve welcome page css and table cell aignments.
Use released rails-html-sanitizer and rails-deprecated_sanitizer
Revert "Revert "Merge pull request #16888 from jejacks0n/render_template""

This reverts commit 585e75696b31395aee895e5366e331c07c3f5ee1.
Fix actionpack test cases broken by #16888
Fix string/gid collision in job arguments

Serialize Global IDs as special objects, distinguishable from Strings
Merge pull request #17024 from buddhamagnet/amend-welcome-page

Amend welcome page
:nail_care: the CSS
Remove unused code