A pass over the Getting Started guide sections 1, 2, and 3 [ci skip]

Upgraded jquery-rails gem version

This is a follow up to: https://github.com/rails/rails/commit/07c70245a128cfe42f134be8759963dc98f1a63e

As suggested by @fxn this comment should not be visible in the API:
https://github.com/rails/rails/commit/07c70245a128cfe42f134be8759963dc98f1a63e#commitcomment-5331658

Add missing directory slashes [ci skip]

Fix tidy_bytes for JRuby

This was a reaction to: https://github.com/rails/rails/pull/9700#issuecomment-34550210

from https://github.com/rack/rack/blob/master/lib/rack/server.rb#L289-L300.

The previous implementation was broken because JRuby (1.7.10) doesn't
have a code converter for UTF-8 to UTF8-MAC.

Closes #13993.

`Rails.threadsafe!` mode is deprecated, Update Docs [ci skip]

* Add note about managing Ruby environments
* Point to curated lists of Ruby learning resources
* Expound on DRY and Convention over Configuration
* Remove note on superuser--thankfuly coders new to Rails don't have to sudo anymore :)
* Installation/verification instructions for SQLite3, since it's a dependency

[ci skip]

Test coverage improvement

If we set :shallow_path when shallow is called it can result in incorrect
paths if the resource is inside a namespace because namespace itself sets
the :shallow_path option to the namespace path.

We fix this by removing the :shallow_path option from shallow as that should
only be turning shallow routes on and not otherwise affecting the scope.
To do this we need to treat the :shallow option to resources differently to
other scope options and move it to before the nested block is called.

This change also has the positive side effect of making the behavior of the
:shallow option consistent with the shallow method.

Fixes #12498.

The `as` option was already explained in a previous example
and doesn't need to be repeated. Explain only the `locals` option
which the example is meant for.

Fixed typo [ci skip]

Clarifying confusing phrasing

Conflicts:
	guides/source/active_record_validations.md
	guides/source/api_documentation_guidelines.md
	guides/source/configuring.md

Fix response flatten infinite recursion

FilterParameters is referenced at the class level from the Request

Remove outdated TODO from url_for test, this test was fixed in 900a2d30

check it is handles empty statement

from e1e17a55

Closes #9131.

kind of exception we should expect for this internal comment.

Since it's already required in the file, we don't need to use autoload
too. This commit is symmetrical change to 0b101804 for Response.

use File.exist? instead of File.exists?