Add mumbling-spec and expression-spec to website

[manuzhang]

Feature Request / Improvement

Add https://github.com/apache/iceberg/blob/main/format/mumbling-spec.md and in-progress expression-spec (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
• I cannot contribute this improvement/feature at this time

[pvary]

I would wait with this until we finalize, and decide if we want to use it