Document conventions API

[AndriySvyryd]

For users and provider writers.

• ConventionSource
• ConventionContext
• ConventionDispatcher
• ConventionBatch
• ConventionSet
• ConventionsBuilder
• ModelCustomizer
• The dangers of IConvention* API

Move the paragraph about conventions from "Model Bulk Configuration" to the new page and replace it with a link.
Always check whether the entity type is still in the model in OnRemoved conventions

[ajcvickers]

Update https://aka.ms/efcore-docs-conventions when done.