Doc-Builder: Add docs.github.com url to ignore list

This pr try to solve following issue by adding docs.github.com
to ignore list.

```
FILE: ./source/topics/proc_troubleshooting-unknown-issues.adoc
[✓] https://github.com/code-ready/crc/issues
[✓] https://github.com/code-ready/crc/issues/new
[✖] https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/attaching-files

3 links checked.

ERROR: 1 dead links found!
[✖] https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/attaching-files → Status: 403
make: *** [Makefile:138: docs_check_links] Error 123
```

```
$ curl -I https://docs.github.com
HTTP/2 403
x-azure-ref: 0fzhqYgAAAAA4TNP/TuUcR6h8UmZoI8OxRFhCMzBFREdFMDIxNQA1OTZkNzhhMi1jYTVmLTQ3OWQtYmNkYy0wODM1ODMzMTc0YjI=
accept-ranges: bytes
date: Thu, 28 Apr 2022 06:47:27 GMT
via: 1.1 varnish
x-served-by: cache-maa10249-MAA
x-cache: MISS
x-cache-hits: 0
x-timer: S1651128448.577384,VS0,VE51
strict-transport-security: max-age=31557600
```

- github/docs#17358

Author: praveenkumar

images/docs-builder/links_ignorelist.json

@@ -2,6 +2,7 @@
     "ignorePatterns": [
         { "pattern": "^https://api.crc.testing" },
         { "pattern": "^http://api.crc.testing" },
+        { "pattern": "^https://docs.github.com/.*$" },
         { "pattern": "https://console-openshift-console.apps-crc.testing" },
         { "pattern": "http://proxy.example.com" }
     ]