Improve documentation how to write addins

We should improve documentation how to create new addins for Cake. Currently we have some documentation here: https://cakebuild.net/docs/extending/addins/. We should add more detailed documentation how to write addins.

This includes mentioning of https://github.com/WormieCorp/generator-cake-addin and https://github.com/WormieCorp/Cake.Addin.Analyzer.

Also should mention Cake Contrib and guidelines etablished there: https://github.com/cake-contrib/Home/blob/master/Guidelines.md, https://github.com/cake-contrib/CakeContrib.Guidelines

TODO:

- [x] Add best practices
- [ ] Add instructions how to create addin from scratch, including generator options
- [ ] Add documentation about available analyzers

Start with the existing addins documentation at https://cakebuild.net/docs/extending/addins/ and review the linked generator-cake-addin, Cake.Addin.Analyzer, and Cake Contrib guidelines. Done means the page explains creating an addin from scratch, including generator options, and documents the available analyzers alongside the relevant best practices.

Written by the indexing model from the issue text.