Document how to avoid backlinks for references

[mschoettle]

Why:

The existence of redirect.github.com does not seem to be documented.
Yet, several projects, such as dependabot and Renovate Bot, make use of it to avoid backlink spam.

It would be good to have an official documentation for this.

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

Adds a small section to "Auto-linked references and URLs" explaining that redirect.github.com can be used to avoid backlinks.

See for example the discussion in: renovatebot/renovate#10796 (comment) and renovatebot/renovate#30980

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 a 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 preview environment.

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Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

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Source	Preview	Production	What Changed
get-started/writing-on-github/working-with-advanced-formatting/autolinked-references-and-urls.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

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[nguyenalex836]

@mschoettle Thanks so much for opening a PR! I'll get this triaged for review ✨

[raiph]

The comment https://www.github.com/orgs/community/discussions/23123#discussioncomment-3239240 (posted 2.5 years ago) is relevant here because it established:

• The work-around of using the www subdomain. If you're going to document use of redirect for this purpose instead then you may want to consider adding a sentence deprecating use of www for the same purpose.

• Two sub-problems that I presume apply no matter whether the work-around uses redirect or www:

... the pop-up that usually appears when hovering over a link to a GitHub issue or PR ... no longer appears

... GitHub owners could break [these work-arounds] at any time by treating [these altered] links [as if the were the same as https://github.com...] for the purpose of adding cross-links"

You may want to consider establishing whether a reasonably quick resolution, or at least clarification, about one or both of these sub-problems, is a reasonable goal before completing and merging a PR updating the doc.

[nguyenalex836]

Thank you for that additional insight, @raiph! Our writers will be sure to consider this when they review this PR 💛

[github-actions]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert 👀