Adds Powered by orgo <version> to the site footer, linking to
https://gitbay.org/krz/orgo. Every HTML page extends base.html, so all
198 carry it.
orgo exposes no version to templates and [site] in orgo.toml rejects
unknown keys, so the version is substituted into the built HTML by
build.py, which reads it from orgo --version after the build. The
template holds the current version as a seed and fallback for builds that
bypass build.py (orgo serve).
The rewrite runs on every page rather than only the re-rendered ones: the build cache keys on the cache format version, not orgo's release version, so pages carried over from a build by an older orgo would otherwise keep printing it. A build where the note matches nothing exits 1.
Verified: clean prod build (199 pages, 0 unresolved links) substitutes on 198 pages; a page hand-edited to an old version is corrected on a fully-cached rebuild; removing the note fails the build.