Change names of documentation files

[DanielNoord]

• Add yourself to CONTRIBUTORS if you are a new contributor.
• Write a good description on what the PR does.

Type of Changes

	Type
✓	🔨 Refactoring

Description

Follow-up to #6628 (comment) and closes #6646.

[coveralls]

Pull Request Test Coverage Report for Build 2350183740

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.353%

---

Totals
Change from base Build 2346865518:	0.0%
Covered Lines:	16066
Relevant Lines:	16849

---

💛 - Coveralls

[DanielNoord]

To test, these links should work now:
https://pylint--6647.org.readthedocs.build/en/6647/messages/messages_list.html ✅
https://pylint--6647.org.readthedocs.build/en/6647/messages/messages_introduction.html ✅

[DanielNoord]

@Pierre-Sassoulas Failure is due to block-disables. See the comment above.

Rather than trying to fiddle around with redirects on three different levels, let's just keep the old names. Sphinx takes the top header as file name anyway so that's not an issue. And using messages_introduction instead of index isn't really an issue imo. Both indicate that it is the top-level page of the messages section.

See also the issue, which is my rationale for fixing this rather than keeping as is.

[Pierre-Sassoulas]

Make sense. Regarding the block disable, should we merge #6642 ? I don't see why the check fails (it's very hard to see where there was an error in the output).

[DanielNoord]

You can search for broken. That makes the error message linkcheck uses for broken links.