docs: Remove references to header_id.md page.

The page was removed along with the extension in 1127013.

Also ensure any future MkDocs warnings cause the build to fail in the tests
so we don't introduce similar errors in the future.

checkspelling.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 
 echo "Building docs..."
-mkdocs build
+mkdocs build --strict
 if [ $? -ne 0 ]; then
     exit 1
 fi

docs/change_log/release-2.1.md

@@ -43,7 +43,7 @@ release, there are a few backward-incompatible changes to note:
   you should now set them on the class. See the [Library
   Reference](../reference.md) for the options available.
 
-* If you have been using the [HeaderId](../extensions/header_id.md) extension to
+* If you have been using the HeaderId extension to
   define custom ids on headers, you will want to switch to using the new
   [Attribute List](../extensions/attr_list.md) extension. The HeaderId extension
   now only auto-generates ids on headers which have not already had ids defined.

docs/change_log/release-2.6.md

@@ -142,7 +142,7 @@ explanation of the current behavior.
 
 ### HeaderId Extension Pending Deprecation
 
-The [HeaderId][hid] Extension is pending deprecation and will raise a
+The HeaderId Extension is pending deprecation and will raise a
 **`PendingDeprecationWarning`** in version 2.6. The extension will be deprecated
 in the next release and raise an error in the release after that. Use the [Table
 of Contents][TOC] Extension instead, which offers most of the features of the
@@ -153,8 +153,6 @@ defined in the HeaderId Extension should note that those functions are now
 defined in the Table of Contents extension and should adjust their import
 statements accordingly (`from markdown.extensions.toc import slugify, unique`).
 
-[hid]: ../extensions/header_id.md
-
 ### The `configs` Keyword is Deprecated
 
 Positional arguments and the `configs` keyword on the

docs/extensions/index.md

@@ -64,7 +64,6 @@ Extension                            | Entry Point    | Dot Notation
 [Smart Strong]: smart_strong.md
 [Admonition]: admonition.md
 [CodeHilite]: code_hilite.md
-[HeaderId]: header_id.md
 [Meta-Data]: meta_data.md
 [New Line to Break]: nl2br.md
 [Sane Lists]: sane_lists.md

docs/extensions/meta_data.md

@@ -101,9 +101,6 @@ Compatible Extensions
 The following extensions are currently known to work with the Meta-Data
 extension. The keywords they are known to support are also listed.
 
-* [HeaderId](header_id.md)
-    * `header_level`
-    * `header_forceid`
 * [WikiLinks](wikilinks.md)
     * `wiki_base_url`
     * `wiki_end_url`