Update Todos documentation and screenshots to include new functionality.

Updates documentation for todos, includes updated and new screenshots.

See also !4502 and #18906 

Closes #18923

[ci skip]

See merge request !4840

Refactor labels docs

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/18555

See merge request !3347

Add information about manually creating Todos from issue and merge request sidebars.

[ci skip]

Due to https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

[ci skip]

[ci skip]

Documented the up key shortcut

## What does this MR do?

Documents the `up` key edit note shortcut in the `/help/_shortcuts.html.haml` help pane and the `/doc/workflow/shortcuts.png` screenshot.

## Are there points in the code the reviewer needs to double check?

The issue and MR shortcuts should be merged, they're the same? Shall I open an issue or include here?

This panel also need to be a little more responsive. Shall I open an issue or include here?

Additionally, I have some conflicts regarding the screenshot. This screenshot also needs to update for !4570.

## Why was this MR needed?

Everyone should know about this awesome shortcut.

## What are the relevant issue numbers?

Closes #18114.

## Screenshots (if relevant)

View the [updated screenshot](https://gitlab.com/gitlab-org/gitlab-ce/blob/e7221ad66efeb0f7d263ead7ed523d194243fb09/doc/workflow/shortcuts.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- [ ] Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4758

Document access request to project & group

## What does this MR do?

Adds documentation that was missing from !3798.

## Are there points in the code the reviewer needs to double check?

I've taken the opportunity to update the images in the "Project users" documentation page, I can remove the commit if it's not welcome!

## What are the relevant issue numbers?

Closes #18584.

## Does this MR meet the acceptance criteria?

- [ ] Tests
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

/cc @DouweM 

See merge request !4685