docs: move MSSQL_SUPPORT.md into sqlx-mssql crate and fix test coverage table

Relocate documentation to live alongside the driver code. Update the test
coverage table to fix an incorrect path and add missing test file entries.

Author: Pablo Carrera <pabloce@poscye.com>

Author: pabl-o-ce

MSSQL_SUPPORT.md sqlx-mssql/MSSQL_SUPPORT.mdMSSQL_SUPPORT.md renamed to sqlx-mssql/MSSQL_SUPPORT.md

@@ -1184,4 +1184,6 @@ Comprehensive test suite in `tests/mssql/`:
 | Derives | `derives.rs` | `#[derive(FromRow)]`, custom field mappings |
 | Query builder | `query_builder.rs` | Dynamic query construction, parameter handling |
 | Error handling | `error.rs` | Database error inspection, error details |
-| Compile-time macros | `tests/mssql-macros/` | Online and offline macro verification |
+| Compile-time macros | `macros.rs` | Online and offline macro verification |
+| Describe | `describe.rs` | `sp_describe` column metadata and type inference |
+| Migrations | `migrate.rs` | Migration lifecycle: create, run, revert |