Expand Section Links documentation and add a section for Custom Anchors

[dodexahedron]

Why:

The information on section links and how they work doesn't really explain how to use them or how they are made.

Also, custom anchors can be used (and are, in fact, quite common in a lot of Microsoft Learn documents, for precedent), so I added a section for them, as well, in a separate commit.

Closes: #33530

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

As described in #33530, I have expanded and enhanced the Section links reusable, detailing behavior and usage, as well as added a small section detailing behavior and usage of custom anchor tags.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

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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]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax.md	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9
repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-readmes.md	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9	from reusable

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[dodexahedron]

Re-based on master.

If checks pass and it looks good to me in the preview environment, I'll be ready to submit this for review.

[dodexahedron]

Re-pushed from my local environment to sign the commits.

[dodexahedron]

Once more, with feeling...

[dodexahedron]

Those linter errors for broken links don't have anything to do with my changes, as they're in completely unrelated files. Linter does not throw any errors when run locally as npm run lint-content -- --paths .\content\get-started\writing-on-github\getting-started-with-writing-and-formatting-on-github\basic-writing-and-formatting-syntax.md

Here are the previews for the modified and added documentation:

https://docs-33531-114a4a.preview.ghdocs.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#section-links

https://docs-33531-114a4a.preview.ghdocs.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#custom-anchors

This is ready for review.

[dodexahedron]

Hm. I should probably make the section link I added to the #headings section a relative link rather than just an anchor target.

Stand by...

[dodexahedron]

Links are now relative.

Let's see how this goes...

[dodexahedron]

Bah. Checks pass, but I accidentally mixed up a link.

Correction coming up.

[dodexahedron]

Change made and it now looks and behaves as intended.

I also made the following comment in the associated issue, #33530, but I'll paste/quote it here, as well, for visibility:

#33530 (comment)

One thought I had, while going through the style guide, is that the links section, both before these changes, but now especially after them, would match the style guide better if there were a section ToC, as well as if h3 an h4 headers were used. I opted not to implement that guideline, however, as the rest of the containing document does not follow that guideline, either, so it seemed better to conform to the document's de facto style, instead.

[nguyenalex836]

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

[dodexahedron]

My pleasure.

Any requests/thoughts/ideas/rude commentary, just let me know!