[docs] Indicate navigation links that take the user to an external site

### Feature Request / Improvement

The Iceberg docs left-hand nav currently has a mixture of links to within the docs, and those external such as third-party sites.
The UX of this can grate, because the user is never sure when they're leaving the Iceberg docs or not.

We should update the docs so that link use one of the generally recognised icons to indicate an external link such as https://fontawesome.com/v4/icon/external-link

### Query engine

None

### Willingness to contribute

- [x] 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

Start by locating the Iceberg docs left-hand navigation configuration and review how its internal and external links are defined. Update the external entries to use a recognised external-link icon, then verify that external destinations are visibly distinguishable from links within the docs.

Written by the indexing model from the issue text.