Update docs based on customer upgrade from 4.0
## What does this MR do?
Adds helpful notes to early upgrade docs. Without these helpful hints it will be really hard for most people to upgrade from early versions to later versions.
The tip about doing database migrations to version 6.1, then migrating iids, then migrating the database all the way was based on a tip from https://github.com/gitlabhq/gitlabhq/issues/7869. It worked really well!
See merge request !4409
Update Todos documentation and screenshots to include new functionality.
Updates documentation for todos, includes updated and new screenshots.
See also !4502 and #18906Closes#18923
[ci skip]
See merge request !4840
Clarify artifact expiry
## What does this MR do?
Clarifies documentation about artifact expiry.
## Are there points in the code the reviewer needs to double check?
## Why was this MR needed?
## What are the relevant issue numbers?
## Screenshots (if relevant)
See merge request !4831
Document `GIT_STRATEGY` and `GIT_DEPTH`
## What does this MR do?
Documents `GIT_STRATEGY` and `GIT_DEPTH`. Also fixes the TOC since the anchors weren't working.
## Are there points in the code the reviewer needs to double check?
The TOC and the location of the new entries.
## Why was this MR needed?
## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/188
## Screenshots (if relevant)


/cc @axil @ayufan
See merge request !4720
Expand on Award Emoji documentation, update/add screenshots.
Adds documentation for award emoji now that comments can be awarded with emojis.
See also !4291 and #18906Closes#9091
cc: @virtuacreative @axil
See merge request !4839
Add some docs for Docker Registry configuration - Relatively important for anyone installing from source
There have been a few issues created because of the lack of documentation for how to configure Docker Registry when installing from source, specifically what the `realm` needs to be and the `service`
These 2 bits of info will stop a lot of headaches 👍
Issues:
https://gitlab.com/gitlab-org/gitlab-ce/issues/17957https://gitlab.com/gitlab-org/gitlab-ce/issues/17991
See merge request !4738
* Adds pipelines page to CI docs.
* Adds image of pipelines list (although currently missing border that is on other images).
* Changes CI to CI/CD in `/doc` and `/doc/ci`
* Sorts user documentation in `/doc`
Partially fixes#17733.
See merge request !4660
Make it possible to lock runner on a specific project
Make it possible to lock runner on a specific project.

----

----

Closes#3407
See merge request !4093
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
* upstream/master:
Add notification_settings:events to schema
Fixed mobile styling
Set path for pinned nav cookie
Fixed placement of close button on merge requests
using request dot path instead
Turn note into normal text
Document environments and deployments
Tweak grammar
fix double query string in url
Broken instructions. git fetch --all --tags doesn't refresh tags