Fixed Typo in README.md

## What does this MR do?
Found a Typo in the README.md file.  before_script should be with ":"  Script didn't work for me without ":"

## Why was this MR needed?
Missing ":" can be irritating for new users.

See merge request !4419

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)

![image](/uploads/eccfd60199d81e6588c13aab817f0eb5/image.png)

![image](/uploads/bbbffe61db64ef17f1fa52907275f3d4/image.png)

/cc @axil @ayufan 

See merge request !4720

Document environments and deployments

## What does this MR do?

Adds an introduction to environments and deployments and tweaks the `environments` entry in `.gitlab-ci.yml` documentation.

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

## Why was this MR needed?

## What are the relevant issue numbers?

Closes #18683.

## Screenshots (if relevant)

![image](/uploads/1e3355c543abaf7b358f3c37596b1f0b/image.png)

![image](/uploads/58a51505674bcf605039db46833a8ae6/image.png)

/cc @ayufan @axil 

See merge request !4725