fix updating-docs link, add regression test

Author: mark-i-m

src/SUMMARY.md

@@ -82,3 +82,4 @@
 [Appendix B: Background material](./appendix/background.md)
 [Appendix C: Glossary](./appendix/glossary.md)
 [Appendix D: Code Index](./appendix/code-index.md)
+[](./important-links.md)

src/important-links.md

@@ -0,0 +1,12 @@
+# Ignore me
+
+This file is a collection of links that are not link-checked by anyone else,
+but we want them to work. For example, the stabilization guide link is often
+posted to tracking issues on GitHub, which might break later if the guide is
+changed.
+
+[Skip this file. Or read it; I'm book, not a cop.](https://imgur.com/gallery/mSHi8)
+
+[1](https://forge.rust-lang.org/stabilization-guide.html)
+[2](https://rust-lang.github.io/rustc-guide/stabilization_guide.html#documentation-prs)
+[3](https://forge.rust-lang.org/stabilization-guide.html#updating-documentation)

src/stabilization_guide.md

@@ -11,6 +11,8 @@ following steps.
 
 ## Documentation PRs
 
+<a name="updating-documentation"></a>
+
 If any documentation for this feature exists, it should be
 in the [`Unstable Book`], located at [`src/doc/unstable-book`].
 If it exists, the page for the feature gate should be removed.