Extended contribution guide with how to make IDEs recognize Moby’s repository as a Go project
Closes #43451

@vastjs FYI

**- What I did**

In order to help new contributors configure the project for development in the IDE like GoLand, I have extended the contribution guide with a page, describing what to do to make IDE handle the non-standard structure of the Go project in Moby’s repository (i.e. `vendor.mod` instead of `go.mod`).

The new page can also serve as a good place for the potential future instructions, regarding IDE configuration.

**- How I did it**

I have extended the contribution guide with an optional step, between downloading the required software for development and configuring Git. The reason behind putting it there was that the next step (Git configuration) is where the first file modifications are done, so it would be good to have a functioning IDE by that point.

The added guide page describes the idea behind `vendor.mod` file and `vendor.sh` script. It also mentions that executing the script will generate `go.mod`, necessary for IDE like GoLand to properly recognize the cloned repository as a Go project, index the vendor directory and provide code completion for the libraries.

**- How to verify it**

As @vastjs wrote in https://github.com/moby/moby/issues/43451:

In GoLand 2021.3:
1. Clone repository
2. Open directory in GoLand
3. Open arbitrary Go file (eg. `daemon.go`)
4. Dependencies are not resolved, code completion doesn't work properly
5. Execute `./hack/vendor.sh`
6. GoLand indexes the vendor directory and code completion is now working

**- Description for the changelog**

Extended contribution guide with how to make IDEs recognize Moby’s repository as a Go project

**- A picture of a cute animal (not mandatory but encouraged)**

docs/contributing/README.md
### Get set up for Moby development

 * [README first](who-written-for.md)
 * [Get the required software](software-required.md)
 * [Set up for development on Windows](software-req-win.md)
 * [Configure Git for contributing](set-up-git.md)
 * [Work with a development container](set-up-dev-env.md)
 * [Run tests and test documentation](test.md)

### Get set up for Moby development

 * [README first](who-written-for.md)
 * [Get the required software](software-required.md)
 * [Set up for development on Windows](software-req-win.md)
 * (Optional) [Configure project for IDE](set-up-ide.md)
 * [Configure Git for contributing](set-up-git.md)
 * [Work with a development container](set-up-dev-env.md)
 * [Run tests and test documentation](test.md)

IMO we don't need to state that this is "optional" (it's already clear enough, just like setting up for Windows is clearly optional if you don't use Windows)