Add links to zi docs

[forsyth2]

Summary

Objectives:

• Now that Add documentation for zppy-interfaces zppy-interfaces#51 has merged, add links to the new zppy-interfaces documentation to the zppy docs.

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.

[forsyth2]

The latest rendered docs can be found here. I've reviewed the changes there.