Add guidelines to addin best practices for best docs integration

[pascalberger]

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 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 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.