ci: pin docs-engine to v0.4.1 for doc search

[burinc]

What

Moves this project's site build from docs-engine v0.3.0 to v0.4.1. The pin
in .github/workflows/site.yml is the only change.

Why

v0.4.1 adds client-side search to every generated site. A magnifier in the
nav (or /) opens it, and results jump to the matching section of a guide
page, opening a collapsed section if the hit is inside one. The engine writes
a search-documents.json index at build time, and lunr.js plus the index
load only when a reader first opens search, so a reader who never searches
downloads neither. A project can turn it off with :search false in
docs/site.edn.

It also picks up the two releases this repo skipped: v0.3.1 (shareable
heading anchors) and v0.3.2 (jolt run build/clean fix, docs-engine#3).

Verification

Built locally with jolt run build against docs-engine v0.4.1, on a tree
identical to this branch apart from the pin:

• the build completed and wrote a 32-record search index, every href under /ebb/
• docs/check-site.sh passed ("index, guide overview, evaluation, conformance and the ADR all present, every URL under /ebb")

CI on this PR is the authoritative check.

Prior art

• ci: pin docs-engine to v0.3.0 for the default Contributing page #8, the v0.3.0 pin this replaces
• jlt-commons/docs-engine v0.4.0 (search) and v0.4.1 (search-result styling fix)