move "HTTP PATCH" paragraph into "Upgrading 3.2 -> 4.0". [ci skip]
3.2 to 4.0 section in upgrading guide is no longer wip. [ci skip]
create 4.1 release notes guide scaffold. [ci skip]
first pass through CHANGELOGS to extract 4_1_release_notes. [ci skip]

Extract **notable changes**, **deprecations** and **removals** from
each CHANGELOG.

I tried to reference the commits and pull requests for new features
and deprecations.

In the process I also made some minor changes to the CHANGELOGS.

The 4_1_release_notes guide is declared WIP.
delete obsolete KINDLE.md. Closes #13037. [ci skip].
no need to explain bundler and rails new in the release notes. [ci skip]

/cc @fxn
wrap test that changes inflections in with_dup
implement rename_index natively for MySQL > 5.7
Merge pull request #13084 from lihanli/series-singularize-fix

Fix an activesupport inflector test changing the inflector but not reverting all its changes afterwards
Merge pull request #13038 from ccutrer/mysql_rename_index

implement rename_index natively for MySQL > 5.7
The section that mentions after_commit and after_rollback will swallow exceptions should me made clear. Made that section to be a  instead of a regular paragraph.
Merge pull request #13087 from bikramwp/master

Make it prominent that after_commit and after_rollback swallow exceptions.
fix mysql version check in rename_index

also clarify native rename_index support is >= 5.7, not > 5.7
Merge pull request #13093 from ccutrer/fix-mysql-version-check

fix mysql version check in rename_index
Remove `DatabaseStatements#case_sensitive_equality_operator`. It has been deprecated already.
#type_cast - improve performance & readability
Checking no debugger on JRuby

fixes tests for JRuby
updating  options documentation for associations
removed unnecessary test case and improved test case for belongs_to having invalid  options
Merge pull request #13100 from akshay-vishnoi/f-refactor

#type_cast - improve performance & readability
Merge pull request #13099 from kamipo/remove_case_sensitive_equality_operator

Remove `DatabaseStatements#case_sensitive_equality_operator`. It has been deprecated already.