Fix stale devcontainer path references in README.md and FACILITATOR_GUIDE.md

[shawnewallace]

Summary

README.md and docs/FACILITATOR_GUIDE.md reference stale/incorrect devcontainer paths that no longer exist. The actual directories under .devcontainer/ use a -participant suffix (and the "bilingual" full-toolchain container is actually named maintainer/), but several doc references still use the old short names.

Actual directories (current)

.devcontainer/
  angular-participant/
  dotnet-participant/
  javascript-participant/
  kotlin-participant/
  maintainer/
  python-participant/
  springboot-participant/

Stale references found

• README.md:82 — .devcontainer/dotnet/ (should be .devcontainer/dotnet-participant/)
• README.md:83 — .devcontainer/springboot/ (should be .devcontainer/springboot-participant/)
• README.md:84 — .devcontainer/bilingual/ (should be .devcontainer/maintainer/)
• README.md:112-114 — same three stale paths in the "Getting Started" selection steps
• README.md:525-527 — same three stale paths in a later steps list
• docs/FACILITATOR_GUIDE.md:56-58 — .devcontainer/dotnet/, .devcontainer/springboot/, .devcontainer/bilingual/
• docs/FACILITATOR_GUIDE.md:451 — .devcontainer/springboot/ and .devcontainer/bilingual/ in a troubleshooting tip

Impact

Participants/facilitators following these docs literally (e.g. copy-pasting a path, or looking for a folder by that exact name) will not find the referenced devcontainer directory, since VS Code's "Reopen in Container" picker lists configs by their name field anyway, but any doc reader manually browsing .devcontainer/ for these paths will hit a dead end.

Suggested fix

Do a repo-wide find/replace in README.md and docs/FACILITATOR_GUIDE.md:

• .devcontainer/dotnet/ → .devcontainer/dotnet-participant/
• .devcontainer/springboot/ → .devcontainer/springboot-participant/
• .devcontainer/bilingual/ → .devcontainer/maintainer/

Also worth a quick sanity grep across all docs (docs/**/*.md) for the same three stale path fragments in case other files have the same issue, since this predates the Angular/JavaScript/Python/Kotlin track work and wasn't caught during those PRs either.