#244 docs: mark the ClickHouse and MongoDB dialects as unimplemented

[RAprogramm]

Closes #244

Both dialects were listed as planned in the feature table and the dialect table, which reads like something a user can switch on. Selecting either one is a compile error, and migration generation for them returns nothing.

• The README feature table and the dialect table in all five wiki languages now say not implemented — compile error, with a link to the tracking issue.
• The crate-level attribute reference says Postgres is the only implemented dialect instead of listing three as if they were alternatives.
• The compile_error! messages point at Implement ClickHouse dialect #23 and Implement MongoDB dialect #24 so a user who tries lands on the issue to follow.
• The feature declarations in Cargo.toml explain why the features exist at all.

No dialect is implemented here — that is #23 and #24. This is about not advertising what does not exist.