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

[sobolevn]

Bug report

There are multiple problems with it:

1. It mentions outdated Python version:

   cpython/Lib/idlelib/help.html

   Line 8 in 0335662

   <title>IDLE &#8212; Python 3.13.0a2 documentation</title><meta name="viewport" content="width=device-width, initial-scale=1.0">

   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 #120338 (review)

Linked PRs

• gh-120360: Add self as IDLE doc owner #120571
• [3.13] gh-120360: Add self as IDLE doc owner (GH-120571) #120576
• [3.12] gh-120360: Add self as IDLE doc owner (GH-120571) #120577

[nineteendo]

Hmm, this change was previously rejected:

Remove idlelib/help.html from this PR. The change is wrong.

[terryjreedy]

1. The viewer only displays the text itself, as seen online, and skips some html header and footer lines. I plan to move the skipping code from the viewer to the copier so only the part viewed gets checked in. This will eliminate 'mid-dated' comments. (This may be part of some issue or draft issue.)
2. As long as .rst changes are propagated to help.html in the same PR, nothing is needed on release.
3. So checking that help.html is updated when idle.rst is would be excellent. I have no idea how to do that. Do you, Nikita?
4. I am thinking about what footer to add.
5. What is the effect of adding something to .getatttributes?

[terryjreedy]

2. To aid propagation, PR adds me as idle.rst codeowner.