Document that unsupported HTTP methods return 404, not 405

[jornith]

Why:

Closes: #45184

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adds a paragraph to the "404 Not Found for an existing resource" section of the REST API troubleshooting guide: a request that uses an HTTP method the endpoint does not support also returns 404 Not Found (rather than 405 Method Not Allowed), so a wrong verb can look like a missing resource. The paragraph follows the existing "check your URL for typos" guidance and points readers to the endpoint reference for supported methods.

Verified against the live API (repro: test_unsupported_method_is_masked_as_404 in https://github.com/jornith/octoprobe).

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Source	Review	Production	What Changed
rest/using-the-rest-api/troubleshooting-the-rest-api.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

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