Prevent exception of loading database yaml with blank config file

Most of this section was written from the time that render file: was the
default, before CVE-2016-0752.

This updates the guide to the Rails 6 `render file:` behaviour, moves it
to a more appropriate part of the file.

[ci skip]

Most of these examples no longer work. The ability to render an absolute
path was removed as part of CVE-2016-0752. In Rails 6 we changed `render
file:` to only take exact paths and always to render them as raw files.
Rendering a template with its extensions _is_ supported, but I don't
think should be recommended.

This also removes `render file:` from an example where it isn't
relevant.

[ci skip]

All this has been simplified.

Merge pull request #36647 from giraffate/fix_exists_with_distinct_and_offset_and_order_in_postgresql

Fix `relation.exists?` with giving `distinct`, `offset` and `order` for joined table

Share the Arel dispatch cache between connections

Share the column and table name quote cache between connections

update example url for add_source

[ci skip]

MySQL: Check error number instead of a message

Fix query cache when using shared connections

Make "bin/setup" works when using PostgreSQL with locales other than en locale

Fix Identation

[ci skip]

ref https://github.com/rails/rails/pull/36621#discussion_r301208961

(Backport 6-0-stable) Add database_exists? method to connection adapters

When SQLite connects it will silently create a database if the database does not
exist. This behaviour causes different issues because of inconsistent behaviour
between adapters: #36383, #32914. This commit adds a `database_exists?` method
as a way to check the database without creating it. This is a stepping stone to
fully resolving the above issues.

When DATABASE_URL is specified don't trample envs that use a url: key

Do not use aliases in GROUP BY clause

MySQL: Fix schema dumping `enum` and `set` columns correctly

Fixes that file attachments without captions would not be represented in plain text generated from rich-text content, causing ActionText::RichText#present? to return false.

Closes #36607.

Fix Browser#preload for older Selenium

Preload browser driver_path to fix system testing under parallelism

Unlock `selenium-webdriver` gem version

This reverts commit ceaeac8c.

Preload browser driver_path to fix system testing under parallelism

Fix swapped replica/primary in multi-db docs [ci skip]

Add the ability to set the CSP nonce only to the specified directives

This version makes eager loading and autoloading consistent,
as documented in the upgrading guide.

Fix programmatic clicks with data-remote

Fix broken url configs

Warn if we can't read the yaml to create database tasks

Replace the www.robotstxt.org URL with https one [ci skip]

[ci skip] Refer actionmailbox and actiontext in the guide

Merge pull request #36210 from vishaltelangre/raise-record-invalid-when-associations-fail-to-save-due-to-uniqueness-failure

Fix: ActiveRecord::RecordInvalid is not raised when an associated record fails to #save! due to uniqueness validation failure