Add type mapping section to TPCH connector

[jhlodin]

Description

Adds a type mapping section to the TPCH connector doc. All connector docs should have type mapping sections for consistency, even when the contents are limited.

Is this change a fix, improvement, new feature, refactoring, or other?

Improvement

Is this a change to the core query engine, a connector, client library, or the SPI interfaces? (be specific)

Documentation

How would you describe this change to a non-technical end user or system administrator?

Describe why type mapping does not apply to the TPCH connector

Related issues, pull requests, and links

Documentation

( ) No documentation is needed.
(x) Sufficient documentation is included in this PR.
( ) Documentation PR is available with #prnumber.
( ) Documentation issue #issuenumber is filed, and can be handled later.

Release notes

(x) No release notes entries required.
( ) Release notes entries required with the following suggested text:

# Section
* Fix some things. ({issue}`issuenumber`)