docs: ask MkDocs to fail on links to non-existent sections

This also requires anchors to be lower-cased: mkdocs/mkdocs#3703

docs/design/run.md

@@ -9,7 +9,7 @@ Initial Version, 10.12.2022 (view full history [here](https://docs.google.com/do
 Jujutsu which will be used to seamlessly integrate with build systems, linters
 and formatters. This is achieved by running a user-provided command or script 
 across multiple revisions. For more details, read the 
-[Use-Cases of jj run](#Use-Cases-of-jj-run).
+[Use-Cases of jj run](#use-cases-of-jj-run).
 
 ## Preface
 
@@ -229,7 +229,7 @@ command)
 (may alias another command)
 * --clean, remove existing workspaces and remove the ignored files
 * --readonly, ignore changes across multiple run invocations
-* --error-strategy=`continue|stop|fatal`, see [Dealing with failure](#Dealing-with-failure)
+* --error-strategy=`continue|stop|fatal`, see [Dealing with failure](#dealing-with-failure)
 
 ### Integrating with other commands
 

docs/github.md

@@ -245,7 +245,7 @@ the [tutorial][tut].
 [detached]: https://git-scm.com/docs/git-checkout#_detached_head
 [gh]: https://docs.github.com/en/authentication/connecting-to-github-with-ssh/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent
 [http-auth]: https://github.com/martinvonz/jj/issues/469
-[tut]: tutorial.md#Conflicts
+[tut]: tutorial.md#conflicts
 [stacked]: https://jg.gg/2018/09/29/stacked-diffs-versus-pull-requests/
 
 ## Using several remotes

docs/glossary.md

@@ -115,7 +115,7 @@ See [visible commits](#visible-commits).
 
 ## Operation
 
-A snapshot of the [visible commits](#visible-commits) and [branches](#branches)
+A snapshot of the [visible commits](#visible-commits) and [branches](#branch)
 at a given point in time (technically a [view object](#view)), together with
 some metadata. The metadata includes the username, hostname, timestamps, and
 pointers to the operation's parents.

mkdocs.yml

@@ -32,6 +32,9 @@ extra:
   version:
     provider: mike
 
+validation:
+  anchors: warn
+
 # IMPORTANT: any changes to plugins have to be duplicated in
 # `mkdocs-offline.yml`. See that file for more details.
 plugins: