Add guidelines to addin best practices for best docs integration

We currently already have a short section how to document addins: https://cakebuild.net/docs/extending/addins/best-practices#documentation

`AliasCategory` tags are only used for integrating documentation on website and we should enhance the best practices with guidelines how we expect categories to be set for best integration into https://cakebuild.net/dsl/ and how NuGet package tags should be set for best integration into https://cakebuild.net/extensions/.

The basic idea of current website is to have an [Extensions page](https://cakebuild.net/extensions/) which lists all extensions (currently addins & modules). On every extension page we list all aliases provided by the specific extension (e.g. https://cakebuild.net/extensions/cake-azuredevops/). Extension page allows to filter on extension type, name and tags.
The [reference page](https://cakebuild.net/dsl/) lists all aliases, built-in and aliases provided by addins, grouped by category. It allows to filter the categories on name. There is also a full text search available across all aliases.