ci: pin docs-engine to v0.4.3 (escape bare <)

[burinc]

What

Moves this project's site build from docs-engine v0.4.2 to v0.4.3. The pin
in .github/workflows/site.yml is the only change.

Why

markdown-clj passes every bare < straight into the HTML, so a browser
opens a tag at something like x<y and hides everything up to the next
>. A sentence such as "compare x<y and stop." showed as just "compare x".
v0.4.3 escapes each < that doesn't begin real HTML, the way GitHub does,
and leaves real tags, comments, and <script>/<style> bodies alone.

Building all fifteen jlt-commons sites before and after changed exactly one
page anywhere (three spaced < in raylib-ios-demo that already displayed
correctly), so this is protection for future pages rather than a fix to a
visible one here.

Verification

A fresh jolt run build of this tree against docs-engine v0.4.3 completed,
and 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 v0.4.2 (search Tab-out, untitled pages) #10, the v0.4.2 pin this replaces