Single source of truth for merge requests docs

Closes #61516

See merge request gitlab-org/gitlab-ce!29269

- Edit pages under user/project/merge_requests and
  add the relevant types in the frontmatter.
- Clean up descriptions.

Based on https://gitlab.com/groups/gitlab-org/-/epics/1280

Revert "Merge branch '50070-legacy-attachments' into 'master'"

See merge request gitlab-org/gitlab-ce!29391

This reverts commit fd19f887, reversing
changes made to abb2d4c6.

Bump sentry-raven to 2.9

See merge request gitlab-org/gitlab-ce!29352

Remove example.net, replace with example.com

See merge request gitlab-org/gitlab-ce!25535

# Conflicts:
#   doc/user/project/clusters/serverless/index.md

Align system note within discussion with other notes

Closes #55033

See merge request gitlab-org/gitlab-ce!29367

Externalize profiles preferences

Closes #53349

See merge request gitlab-org/gitlab-ce!28470

Resolve "Merge request discussion text jumps when resolved"

Closes #61565

See merge request gitlab-org/gitlab-ce!28995

changed dropdown styles in board page

See merge request gitlab-org/gitlab-ce!29338

Remove list sorting from NewListDropdown

See merge request gitlab-org/gitlab-ce!29077

Show description of 'gettext:regenerate' task

See merge request gitlab-org/gitlab-ce!29362

Add a note about nested scopes matching

See merge request gitlab-org/gitlab-ce!28466

Backport of EE Displaying Blocking MRs

See merge request gitlab-org/gitlab-ce!29095

Move issue board models to boards directory

Closes #61293

See merge request gitlab-org/gitlab-ce!29260

Some classes in vue_shared were used only by boards, so they were
moved to the boards directory.

Allow masking if 8 or more characters in base64

See merge request gitlab-org/gitlab-ce!29143

Fix migration specs using factories

Closes #62911

See merge request gitlab-org/gitlab-ce!29350

We need to stub default_git_depth and default_git_depth= because some
old migrations specs try to create a record using schema before that
column was introduced.

The `let!` calls were executed before the `before` hook which still
caused some factories to fail, so they're created in the `before` hook
now as well.

Add task count and completed count to responses of Issue and MR

See merge request gitlab-org/gitlab-ce!28859

Improve email text part

See merge request gitlab-org/gitlab-ce!29111

Remove extraneous DiffNote#supports_suggestion? calls

See merge request gitlab-org/gitlab-ce!29027

Add project level git depth setting

Closes #59688

See merge request gitlab-org/gitlab-ce!28919

Migrate legacy uploads

Closes #57217

See merge request gitlab-org/gitlab-ce!24679

Add dependency list documentation

See merge request gitlab-org/gitlab-ce!29307

[CE of Part 2] 5276 Update existing stores for file terminal sync

See merge request gitlab-org/gitlab-ce!29333

[CE of Part 1] 5276 Remove 'build-page' from 'ide-terminal' element

See merge request gitlab-org/gitlab-ce!29331

Add spec for task_completion_status

Add test cases for task_completion_status result

Extracted shared samples

Add new spec file for task completion status response

Fix style errors

Add changelog entry

Changed samples to Hashes

Remove test for successful request

Remove not nil expectation

Add task_completion_status to api documentation for issues

Add task_completion_status to api documentation for merge_requests

Refactor spec so it just requests one specific item

Add task_completion_status to Taskable

Simplified task completion status in entities

Refactor spec so it separates status code check and content check

Fix spec description text and field name

added code differences from EE to CE

Closes gitlab-ee#10088

See merge request gitlab-org/gitlab-ce!29271

Remove metrics_time_window feature flag

Closes #62091

See merge request gitlab-org/gitlab-ce!28670