docs: update first-project guide for new deno init template (2.8)

[lunadogbot]

Summary

Updates the "Making a Deno project" walkthrough to match the new default deno init template shipping in Deno 2.8 (denoland/deno#33042). The template now scaffolds a small Deno.serve HTTP server (with two tests) instead of the old add(2, 3) math example.

• Rewrites the description of main.ts and the run/test output blocks in runtime/getting_started/first_project.md.
• Mentions the import.meta.main guard so readers understand why the tests import the handler directly.

Test plan

• deno task serve — page renders.
• If reviewers want me to verify the exact test names against the upstream template, I can adjust before merge.

[fibibot]

Two generated-template details do not match merged denoland/deno#33042, so readers would copy commands/output that the 2.8 deno init template does not produce.

1. $ deno main.ts should be deno run --allow-net main.ts; upstream init prints that command, and the scaffolded Deno.serve(handler) path needs net permission.
2. handler returns hello / handler returns 404 for unknown route should match the generated main_test.ts names: returns html on / and returns json on /api. The template does not generate a 404 test.

• nit: bump last_modified; this is a substantive content update.