Test link checker on links in markdown codeblock

[kilfoyle]

This tests three raw link formats inside a markdown codeblock:

[source, markdown]
----------------------------------
I am a humble markdown codeblock.

TEST 1 - Here is a URL in mid-sentence:
This investigation guide uses https://www.elastic.co/guide/en/security/current/osquery-placeholder-fields.html to dynamically pass alert data into Osquery queries.

TEST 2 - Here is a URL with no following text:
This investigation guide uses https://www.elastic.co/guide/en/security/current/es-ui-overview.html

TEST 3 - Here is a URL demarkated by single quotes:
This investigation guide uses 'https://www.elastic.co/guide/en/security/current/security-assistant.html'
----------------------------------

Interestingly, the URL renders much more nicely if we wrap it inside single quotes (Test 3), but I don't know which of these will pass our docs build CI check.

---

And the results are in:

2024-02-23 05:36:59 EST | INFO:build_docs:Bad cross-document links:
-- | --
  | 2024-02-23 05:36:59 EST | INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/fleet/master/fleet-troubleshooting.html contains broken links to:
  | 2024-02-23 05:36:59 EST | INFO:build_docs:   - en/security/current/security-assistant.html'
  | 2024-02-23 05:37:26 EST | 🚨 Error: The command exited with status 255

Only Test 3 failed, so it seems the link should pass CI checks as long as there is whitespace after the .html. So I guess the full URL must be surrounded, before and after, by whitespace.

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[kilfoyle]

Test complete.