Add best-practices for documenting extensions

We should add best-practices how to document extensions for best integration with Cake website.

Points to consider:

* Add XML comments
* Summary on aliases class
* Examples for aliases
* Category on class and aliases
* Tags on NuGet packages
* Add XML documentation files to NuGet packages

Start by reviewing the requested best-practice areas: XML comments for aliases and categories, NuGet package tags, and XML documentation files in packages. Done means the Cake website contains clear guidance covering each listed point; the issue names no specific file, test, or entry point.

Written by the indexing model from the issue text.