Update contributing documentation

## Documentation request

### Edit existing content 📝

**Which page(s) do you want us to edit?**
Multiple; everything related to contributing

**What do you think should be changed?**
- Decide whether the contributing documentation should be written wrt the `master` branch or the `develop` branch.
- If `master`, modify deployment scripts. They currently clone `develop`.
- In either case, go through the content and links, and check that everything makes sense and is relevant.
- Make sure that all content related to contributing is solely under the contributing section. (At the time of writing this, [this](https://techdocs.akamai.com/infection-monkey/docs/scanners#adding-a-scannerfingerprinter) exists which is in the wrong place.)