Adding documentation for custom function mapping

[maumar]

Fixes #500

[roji]

Pedantic English review FTW! Feel free to ignore points if disagree.

BTW see build warnings in the CI checks.

[smitpatel]

Also add

• The HasFunction overload which takes lambda with default params to register a function
• DbFunctionAttribute and properties over that.
• Specifying a schema or marking function as built-in
• Mapping function return type or parameter to particular store type when not natively supported and scenario where it works.
• argument propagates nullability API and link to the null semantics docs.
• Ideally, we should put down the example of mapping to JSON function How to write DbFunction's translation? efcore#11295 (comment) Probably most used usecase for the UDF so far
• Mention that TVF cannot use HasTranslation API
• Few more cautions about HasTranslation API and need to understand the SQL expression tree.

[smitpatel]

Consider translating to ABS(a - b) to avoid this warning and also give example of SqlFunctionExpression translation.

[maumar]

ended up going with slightly more complicated translation to both show the SqlFunction and custom specification of type mappings - should we add a note/caution that this is an advanced scenario?

[smitpatel]

You can avoid type mapping and associated warning if you make first/second double in C# and just reuse their type mapping.

[maumar]

don't we want to show people how to create the mapping themselves? like when you divide/multiply by a double constant, you would have to do it anyway no?