Adds the Standard ML community extension
([omarjatoi/zed-sml](https://github.com/omarjatoi/zed-sml)) to
`docs/src/languages.md`. It is already published in the Zed extension
registry ([zed.dev/extensions/sml](https://zed.dev/extensions/sml)) and
provides Tree-sitter highlighting plus Millet LSP integration, but is
currently missing from the language documentation page.

Release Notes:

- N/A