chore: ignore built docs output and editor backups

dist/ and .astro/ are a built documentation site and its build cache. They are
not generated from this repository -- there is no package.json or astro config
here -- so they cannot be regenerated from it either, and 5.4 MB of stale HTML
does not belong in a Java repository. Ignoring them also stops a careless
`git add -A` from sweeping them into an unrelated commit.

Author: rahlk

.gitignore

@@ -200,3 +200,10 @@ etc/
 # Ad-hoc analysis output from manual v1/v2 comparison runs
 output/
 node_modules
+
+# Built documentation site and its build cache (generated elsewhere, not from this repo)
+dist/
+.astro/
+
+# drawio editor backups
+*.drawio.bkp