Website theme only changes are ignored for deployment

[yves-da]

Problem

This PR #754 changed only content in the theme directory, but that change did not get pushed to docs.daml.com.

Workaround

The theme changes were then pushed to docs.daml.com by means of small documentation changes, see #755 and #757.

Analysis

On CI, the push_docs job didn't deploy to docs.daml.com with the following output:

2.10.0 is up-to-date, nothing to do.
2.3.20 is up-to-date, nothing to do.
2.7.9 is up-to-date, nothing to do.
2.8.9 is up-to-date, nothing to do.
2.9.0 is up-to-date, nothing to do.
3.1 is up-to-date, nothing to do.
draft is up-to-date, nothing to do.
root is up-to-date (2.8.9, a4fb2f8e136bc6811883dd3d8c3f23251a8268ba).
snapshots.json is up-to-date.
versions.json is up-to-date.
/downloads up-to-date, nothing to do

The push_docs job has got a change detection logic which does not trigger on theme directory only changes.
https://github.com/digital-asset/docs.daml.com/blob/main/.circleci/config.yml#L163

Solution idea

Add the theme to the sha logic, that is don't take the latest commit that touched a folder, but the latest that touched a folder or theme.