[skip changelog] Disable internal anchor checks by link checker tool (arduino#1692)

Validation of links to internal anchors was recently added to the markdown-link-check tool used to check for broken
links in this project's documentation.

Although a much needed feature, unfortunately it does not currently support anchors created by HTML anchor tags.

For example, this is valid and common Markdown:

[click here](#some-anchor)
<a name="some-anchor"></a>

but will fail the check:

ERROR: 1 dead links found!
[x] #some-anchor -> Status: 404

This type of link markup is used by the protoc-gen-doc tool that generates the gRPC interface documentation, which
causes a spurious failure of the link check.

The solution is to configure markdown-link-check to skip these links (which was the behavior anyway with versions 3.8.7
and older). That will be reverted whenever the tool is able to correctly validate internal anchor links.

Author: per1234

.markdown-link-check.json

@@ -3,6 +3,9 @@
   "retryCount": 3,
   "aliveStatusCodes": [200, 206],
   "ignorePatterns": [
+    {
+      "pattern": "^#"
+    },
     {
       "pattern": "https?://localhost:\\d*/"
     },