release/: the newest stage's manufacturing files and documentation

A stage tag (v*) now builds, with kicad-cli, pandoc and pdflatex in the CI
image (.gitea/scripts/release-files.py):
- per board: Gerbers + Excellon drill zip (JLCPCB-compatible settings), BOM
  (grouped, DNP left out), pick and place, schematic and PCB PDFs, 3D renders;
- docs/*.md and docs/tex/*.tex as PDF, tag and commit in the footer;
- release/README.md with tag, commit, ERC/DRC result, files, SHA-256 and the
  stage's CHANGELOG section.
Every file name carries the tag. release-attach.py attaches the files plus a
zip of the whole folder to the tag's release (replacing assets of an earlier
run); for the newest stage tag (not older stages, not pre-releases) the job
commits them to release/ on main with [skip ci].

.gitignore: !/release/ after Release/ (Windows matches it case-insensitively);
.gitattributes: release/** -text so the SHA-256 match on every checkout.
TEMPLATE.md: "Release files" section, rules and CI updated.

Tested in git.b4l.co.th/b4l/kicad10_auto_node with harNES's board and docs:
v0.1 committed, v0.0 and v0.2-rc.1 left release/ alone, v0.2 tagged on an
older commit landed on top of a moved main; uploads byte-exact against a stub
of the Gitea release API.

README.md

@@ -7,6 +7,7 @@ TEMPLATE.md explains the layout, the rules and `proplan.toml`.
 
 | Folder | What goes there |
 |---|---|
+| `release/` | **the latest stage's files**: Gerbers, BOM, pick and place, PDFs, renders and documentation; written by CI at every stage tag, never by hand |
 | `assets/` | the product image (`assets/product.png`, shown by proplan), renders |
 | `hardware/$REPO_NAME/` | the KiCad project of the board |
 | `hardware/lib/` | symbols, footprints and 3D models shared by the boards |
@@ -20,9 +21,11 @@ TEMPLATE.md explains the layout, the rules and `proplan.toml`.
 
 ## Prototype stages
 
-Each prototype is a git tag (`v0.1`, `v0.2`, ...). CI builds gerbers, BOM and
-schematic PDFs for the tag and attaches them to its release; CHANGELOG.md says
-what each stage changes. In proplan the prototype stage links the tag.
+Each prototype is a git tag (`v0.1`, `v0.2`, ...). For the tag, CI builds the
+Gerbers and drill files, BOM, pick and place, schematic and PCB PDFs, 3D renders
+and the documentation as PDF, attaches them to the tag's release and puts them
+into `release/`, which always holds the newest stage. CHANGELOG.md says what
+each stage changes. In proplan the prototype stage links the tag.
 
 ## Versioning