Add mumbling-spec and expression-spec to website

### Feature Request / Improvement

Add https://github.com/apache/iceberg/blob/main/format/mumbling-spec.md and [in-progress expression-spec](https://github.com/apache/iceberg/pull/16652) (after it's merged) to https://iceberg.apache.org/terms/.

### Query engine

None

### Willingness to contribute

- [ ] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [x] I cannot contribute this improvement/feature at this time

Start with the website's terms page and the referenced format/mumbling-spec.md; check how existing specifications are included there. Wait for expression-spec in pull request #16652 to merge, then add both specifications to the terms page and verify that their links and rendered content are available.

Written by the indexing model from the issue text.