Add test to verify `idlelib/help.html` is in sync with docs

Bug report

There are multiple problems with it:

1. It mentions outdated Python version: https://github.com/python/cpython/blob/0335662fe1f663fe96e3e4acf0f34c5959d06b00/Lib/idlelib/help.html#L8 Right now it should be Python 3.14a0
2. Is is not rebuilt / executed during release process
3. It is not executed to check that generated files are up-to-date with the source ones (Doc/library/idle.rst in this case) in CI
4. help.html does not contain a comment that it is generated (and from what)
5. help.html is not marked as generated in .gitattributes

CC @terryjreedy

Found while reviewing https://github.com/python/cpython/pull/120338#pullrequestreview-2111016950

Linked PRs

• gh-120571
• gh-120576
• gh-120577

Inizia con Lib/idlelib/help.html e la relativa sorgente, Doc/library/idle.rst, quindi esamina le PR collegati gh-120571, gh-120576 e gh-120577. Determina se il test di sincronizzazione e i controlli sui file generati sono già coperti; il lavoro è terminato quando il file della guida generato viene convalidato rispetto alla sua sorgente e vengono colmate le lacune del release o della CI.