docs: add source-aligned examples and compatibility workflow

[vishr]

Summary

First Echo-side slice of labstack/echox#448. The Astro site stays in labstack/echox.

• Add a nested examples module with complete Request Logger and Static programs that compile against the Echo checkout containing them.
• Add a source-data command that emits exported middleware *Config field names, types, deprecation markers, and source positions as JSON. It currently finds 22 config types; it does not generate runtime behavior claims.
• Add CI that compiles the source-owned examples, keeps the generated config manifest as an artifact, compiles the existing echox cookbook against the proposed Echo checkout, and builds the current Astro site.
• Record the first gomarkdoc trial and its limits for page-specific reference sections.

Verification

• go test ./... at the Echo root: 2,209 tests passed.
• go test ./... in docs/examples: passed.
• echox go test ./... using a temporary Go workspace with this Echo checkout: passed.
• echox/site npm run build: 301 pages built.
• actionlint .github/workflows/docs.yml: passed.

Scope of this draft

The current echox pages still contain known Request Logger and Static drift. This PR prepares source data and cross-repository builds; the next echox change will embed the compiled examples, consume the source manifest, and enforce page-level drift checks. No site files or deployment settings move into Echo.

[aldas]

@vishr, please do not add these "examples" here. even with modules. all these add noise.

[aldas]

expecially if there are additional libraries in these new modules which we need to keep up with the security issues etc.