docs: warn that predefined errors are no longer *HTTPError in v5

[ahashim]

API_CHANGES_V5.md documents the new HTTPError shape and the swapped HTTPErrorHandler signature, but not that the predefined errors (echo.ErrNotFound, echo.ErrMethodNotAllowed, ...) are no longer *echo.HTTPError.

That gap bit us in production after migrating an app to v5.3.1. A common v4 custom error handler does:

if he, ok := err.(*echo.HTTPError); ok {
    code = he.Code
} else {
    code = http.StatusInternalServerError
}

In v5 the router returns unexported sentinels that only implement HTTPStatusCoder, so the assertion never matches and every unmatched route renders as a 500 instead of a 404. Nothing fails at compile time or at runtime. We only noticed because scanner traffic showed up in our logs as thousands of 500s.

The sentinel design looks intentional, since it prevents globally mutating echo.ErrNotFound.Message the way v4 allowed, so this PR only adds documentation: a warning in the "HTTPError Simplified" section and a short note in migration step 4, both pointing at echo.StatusCode(err).

If you'd take a code change as well, httpError could implement As(any) bool so that errors.As(err, &he) matches again. I can send that separately if there's interest.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.37%. Comparing base (dcb05f0) to head (5024b6a).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3063      +/-   ##
==========================================
+ Coverage   93.34%   93.37%   +0.02%     
==========================================
  Files          43       44       +1     
  Lines        4735     4783      +48     
==========================================
+ Hits         4420     4466      +46     
- Misses        192      193       +1     
- Partials      123      124       +1     

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