Prepare bug report templates for Rails 6.0 development

Add missing `require "active_support"` in `guides/bug_report_templates/generic_gem.rb`

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

Closes #31998

Add `:nodoc:` to `ActiveJob::Serializers`
Add `:doc:` to `ActiveJob::Serializers::ObjectSerializer#klass`
Express `ActiveJob::Serializers::ObjectSerializer#klass` as private method

Provides both a forked process and threaded parallelization options. To
use add `parallelize` to your test suite.

Takes a `workers` argument that controls how many times the process
is forked. For each process a new database will be created suffixed
with the worker number; test-database-0 and test-database-1
respectively.

If `ENV["PARALLEL_WORKERS"]` is set the workers argument will be ignored
and the environment variable will be used instead. This is useful for CI
environments, or other environments where you may need more workers than
you do for local testing.

If the number of workers is set to `1` or fewer, the tests will not be
parallelized.

The default parallelization method is to fork processes. If you'd like to
use threads instead you can pass `with: :threads` to the `parallelize`
method. Note the threaded parallelization does not create multiple
database and will not work with system tests at this time.

parallelize(workers: 2, with: :threads)

The threaded parallelization uses Minitest's parallel exector directly.
The processes paralleliztion uses a Ruby Drb server.

For parallelization via threads a setup hook and cleanup hook are
provided.

```
class ActiveSupport::TestCase
  parallelize_setup do |worker|
    # setup databases
  end

  parallelize_teardown do |worker|
    # cleanup database
  end

  parallelize(workers: 2)
end
```

[Eileen M. Uchitelle, Aaron Patterson]

Documents ActiveStorage file cleanup in Integration Tests which is
similar but slightly different than the existing docs for System Tests.

We can speed up things for the supported types by keeping the code in the
way it was.

We can also avoid to loop trough all serializers in the deserialization by
trying to access the class already in the Hash.

We could also speed up the custom serialization if we define the class
that is going to be serialized when registering the serializers, but
that will remove the possibility of defining a serialzer for a
superclass and have the subclass serialized using it.

[ci skip]

[ci skip]

correction to #31755 as per https://github.com/rails/rails/pull/31755#discussion_r165819798

:tada::tada::tada:

Document in the guides the new middleware added in #31162
[ci skip]

I think this is a joke, although not a great one.
It's mildly unprofessional, so I think we
should get rid of it.

Fixes #31703

This was changed with 5d7b70f4 and 428939be.

Because we use `credentials` in an example of configuration files, but
`credentials` can not be used unless it is 1.8.0 or later.

Ref: 9d65ac30

* rDBMS -> RDBMS. There is only place using rDBMS.
* a SQL -> an SQL

* Update active_record_basics.md

I made a bit of clarification for people, who are not familiar with SQL (pretty much like me). However, I don't know what tutorial for MySQL is better, so I haven't inserted a link yet.

* [ci skip] For those who new to GitHub

Added more instructions for contributing guides. Without them, it was somewhat confusing for me to find what I should actually do.

* Update active_record_basics.md

Fixed grammar and text wrapping as requested.

* Update contributing_to_ruby_on_rails.md

Revised instructions.

* Update contributing_to_ruby_on_rails.md

Typos

* Update active_record_basics.md

* [ci skip] Update active_record_basics

Added a few links to SQL tutorials found on the net.
Also, changed MySQL to SQL (or one of its extensions) - I think that it's a good compromise.

* [ci skip] I think now it's more clear what to do.

* [ci skip] Fixed strings

[Rafael Mendonça França + loothunter1]

[ci skip]

The bottom line in the image of `ActionController::UnknownFormat`is not
truncated. Now the text does not say it is.

[ci skip]

Updated the method counts and unified them all on numerals.

Mention `config.generators.system_tests` in
the "Configuring Rails Applications" guide.

  - Remove mention about `ActionView::TemplateHandlers` since this
    module was removed by c1304098.
    Change word `subclasses` to `nested classes`.
    See c7408a0e
  - Remove useless sentence "Beginning with Rails 2, the standard extensions
    are `.erb` for ERB (HTML with embedded Ruby), and `.builder` for Builder (XML generator)."