Add a note about private_profile changes in 12.1

Closes #64613

See merge request gitlab-org/gitlab-ce!30772

Documentations for parallel execution strategy for Merge Trains

Closes #63369

See merge request gitlab-org/gitlab-ce!30661

This commit adds a documentation for parallel pipelines on merge
trains.

Fix typo

Add endsWith to polyfill

See merge request gitlab-org/gitlab-ce!30920

Link Metrics Reports from merge request docs

See merge request gitlab-org/gitlab-ce!30848

Add setting for gitlab-elasticsearch-indexer path

See merge request gitlab-org/gitlab-ee!14728

Use predictable environment slugs

See merge request gitlab-org/gitlab-ce!30551

Removes EE differences for issue boards

See merge request gitlab-org/gitlab-ce!30851

EE: Removes EE differences from issue boards

See merge request gitlab-org/gitlab-ee!14725

Creates auxiliar functions for ee code only

Added new ES rake task and troubleshooting step

See merge request gitlab-org/gitlab-ce!30681

Update lint.md to provide correct path for API call as per #58223

Closes #58223

See merge request gitlab-org/gitlab-ce!25703

Remove duplicate word in Issues feature page

See merge request gitlab-org/gitlab-ce!30919

Document all the available DAST variables

Closes gitlab-ee#10122

See merge request gitlab-org/gitlab-ce!30801

Add docs for GitLab.com subscription upgrades

See merge request gitlab-org/gitlab-ce!30910

Document interacting GitLab components and features

See merge request gitlab-org/gitlab-ce!30813

It's not uncommon that a single code change can reflect and
interact with multiple parts of GitLab codebase, making it
possible to break an existing feature.

This commit expands documentation in that regard for developers.

Set visibility level 'Private' for restricted Internal imported projects

See merge request gitlab-org/gitlab-ce!30522

With https://gitlab.com/gitlab-org/gitlab-ee/issues/12388 change going
live there is potential risk of breaking imports of 'Internal' projects.
This change makes sure if 'Internal' visibility level is restricted
all 'Internal' projects will be marked as 'Private'

See: https://gitlab.com/gitlab-org/gitlab-ce/issues/64311

Update numbered lists and enforce docs standards

See merge request gitlab-org/gitlab-ce!30218

Ensure that all numbered lists use only 1. and no other numbers.
Also ensure that numbered lists use proper spacing.

Add documentation for panel types

See merge request gitlab-org/gitlab-ce!30695