CI(docs): Add ccache to builds

[echoix]

Amongst other things, I explored using ccache for the docs workflow, which builds grass and grass-addons. Since grass-addons don't change often, it has a high chance of benefiting of ccache. It is already used in the grass-addons repo where it provides an small advantage there too.

As expected, most calls can be a cache hit, as shown in a screenshot of a run with a small, non-c change committed after a previous run.

For the time savings, these two show a 2 minute improvement. The time taken and reductions are consistent with what we can see in grass-addons repo.