Docs: Create a page with info on how to do common things

[phansch]

After #3824, it would be nice to continue adding documentation. Specifically, it would be awesome to have a page that explains some common lint writing tools:

• Retrieving the type of an expression
• Retrieving all methods of a type
• Checking if a type implements a specific trait
• Dealing with macros (in_macro, in_external_macro, differing_macro_contexts)
• Checking if a type defines a method or function

If someone wants to start this, it's also fine to just pick only one item from the list and create a new file in the doc/ directory.

[ThibsG]

It remains, at least, how to explain:

• Retrieving all methods of a type

[phansch]

It remains, at least, how to explain:

* [ ]  Retrieving all methods of a type

In retrospect, I think that's a rather useless example. It would make more sense to explain how to check if a type defines a specific method or not. I've updated the issue for that.

[ThibsG]

Yes that makes sense. I will add this in a future PR.