build(docs): fetch tags in readthedocs builds

Readthedocs builds don't fetch tags when cloning the repo. This is a problem
for us because Rockcraft's version comes from the tag (via setuptools-scm),
so do a fetch of the tags as a post-checkout step.

.readthedocs.yaml

@@ -9,6 +9,10 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3.10"
+  jobs:
+    post_checkout:
+      - git fetch --tags --depth 1 # Also fetch tags
+      - git describe               # Useful for debugging
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx: