Add a new Ci::ApplicationRecord that all CI models should inherit from.
This also replaces the usage of the previous Gitlab::Ci::Model.

See: https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html\#please

Changelog: other

Rubocop rule to prevent referencing ActiveRecord::Base.connection.

This page seems to be mostly copied from the same docs about adding
text limits and as such a couple of sentences were still referring to
text limits.

"primary" should be reserved for replication contexts. So we can talk
about main primary, main replica, ci primary, ci replica databases.

We have accidentally caused all CI models to use Ci::ApplicationRecord
when we only want Ci::InstanceVariable to do so

As part of larger effort to move all docs
to index files.

Mostly about node statistics being per-loop averages.

In general type `:text` was used to store encrypted attributes in the
database.

This change recommends to use `:binary`. This makes use of the `bytea`
type in PostgreSQL and makes storage more efficient.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/227779

This change adds migration helper method for renaming a table without
downtime.

For 4 years we've had logic in our database migrations that required you
to set a constant `DOWNTIME = true` if you required downtime and seek
approval from the VP of Engineering. We have never once used this
process as we've always found a way around the problem using a different
approach.

As such we decided in
https://gitlab.com/gitlab-org/gitlab/-/issues/326495 that we should just
remove this `DOWNTIME` constant and the extra checks here to reduce
noise in our database code review and give less processes for people to
learn.

This MR removes a lot of things and here is a high level summary:

1. Remove DowntimeCheck class and the rake task that invoked and the CI
job that invoked that rake task and any related tests, helper classes
2. Update documentation to make it clearer that downtime is not allowed
and therefore remove the approval process
3. Update a page called `what_requires_downtime` to be called
`avoiding_downtime_in_migrations` since it was already a set of patterns
to avoid downtime and now it's worded more strongly to imply that we can
always get away without downtime
4. Various other docs pages that had examples of migrations that used
the `DOWNTIME` constant
5. Various rubocop specs that had migrations in them which used the
`DOWNTIME` constant

The one thing I did not do is go back and remove `DOWNTIME = false` from
all the existing migrations. In general we should not be updating
migrations once they've run and this would have made this MR change many
thousands of files so it's not worth it.

Updates not null constraint documentation to reference the new
high-traffic table definition

Updates all redirecting and 404'ing links in
the /doc/development directory

We should point all links to the latest locations,
or to versioned pages so they don't change,
as needed

Create a cross-link to improve findability.

Whatever's needed: fix spelling, wrap in code tags, add words to
exceptions file, rephrase. Just get it done.

Continuing to clean up spelling issues from our docsets!

Add a new migration helper that allows the caller to define CHECK
constraints in the table definition block. This can be used to avoid
having to `disable_ddl_transaction!` and add the check constraints using
the existing helper, which cannot be used inside a transaction block.
Also provides a convenience method to more easily add CHECK constraints
for text limits.

Replaces some shorthand words to enforce standard

Corrects clear typos in doc/ from Vale warnings

The notation we use to represent a note is:

```
NOTE: **Note:**
This is a note
```

This is rendered by Nanoc and presented in an alert box in the docs
site. The second `**Note:**` can be redundant, so let's remove it.

In https://gitlab.com/groups/gitlab-org/-/epics/5020, we decided to
use only two alerts (note, warning), and eliminate any bold text
coming after the icon.

This changes CAUTION to WARNING and eliminates any
**Warning:**/**Caution:** text that comes after them.

The previous URL had two drawbacks:

1. It didn't match what the title was about (`Assignments to DevOps
   Stages and Groups`, and the anchor was `#designated-technical-writers`).
2. It didn't account for the other sections, like development and API
   guidelines.

These pages contained links or other information that implied a
possible stage / group assignment. This merge request proposes adding
stage / group information to these pages to help with maintenance.

Add a page for query performance guidelines with
timing suggestions and a section on cold and warm
cache.

Link to the new doc from other database doc locations
mentioning query times.

Update the database docs to include instructions on
how to disable an index and use cases.

To standardize the docs, we should make sure to not
use relative links that start with dot slash

Fix this feature-flags page to redirect correctly.