Improve the testing guide [ci skip]

Improve the "Testing Rails Applications" guide adding a section for
testing helpers and a little note about associations with ActiveRecord
and YAML fixtures.

Also add a link to Fixture Builder ; yet another replacement for
fixtures such as Machinist or FactoryGirl.
Added SanitizeHelper to rails guide docs [ci skip]

Added SanitizeHelper to rails guide docs [ci skip]

Added SanitizeHelper to rails guide docs update [ci skip]
Added Action View in 'rake about' [ci skip]
Removing obsolete html tags in favor of markdown code block.
Revert "Removing obsolete html tags in favor of markdown code block"

This reverts commit 157dc275da0ad38635337b7c1d96de656d91de8e.
Minor align BR tags with HTML void - with reference to the generators code change - https://github.com/rails/rails/commit/f038d4cc5fe1ac21f92c7e32c61ff85a0c816f3f
guides should tell people to write ruby code, not output
Merge branch 'master' of github.com:rails/docrails
Merge pull request #11617 from swooop/doc_sanitize

Added SanitizeHelper to rails guide docs [ci skip]
document `assert_not_XYZ` in place of `refute_XYZ`.
Removed redundant asserts for assigns attribute operation
Merge pull request #11886 from jetthoughts/remove_redundant_assert_from_datatype_test

Removed redundant asserts for assigns attribute operation
Rescue invalid ip address exceptions on assign.

In order that set attribute should not be bang method
Merge pull request #11574 from jetthoughts/11552_rescue_on_invalid_inet_assign

Fix assign ip address with invalid values raise exception
Added docs for 'remove' method [ci skip]
Added new line [ci skip]
Typo Fix[ci skip]
Merge pull request #11887 from raysrashmi/typo_fix

Typo Fix[ci skip]
Fixing multi-word automatic inverse detection.

Currently, ActiveRecord models with multiple words cannot have their
inverse associations detected automatically.
Merge pull request #11897 from wangjohn/fixing_multiple_word_inverse_detection

Fixing multi-word automatic inverse detection.