WIP: CI verify yaml in markdown files

What does this MR do?

CI verify each yaml code block in Markdown files via yamllint.

e.g. from https://gitlab.com/gitlab-org/gitlab/blob/master/doc/ci/yaml/README.md

```yaml
job1:
  script: "execute-script-for-job1"

job2:
  script: "execute-script-for-job2"
```

Idea from !35894 (comment 373597038)

a CI script that pulls out all the yaml codeblock, saves them as files and pipes those into yamllint. If you name the created file after the source file + line number, it would be easily findable. Something like YAMLLINT Error in 'source-file-md-line-123.yml', incorrect blah blah blah.

Todo:

• regex to find code blocks
• in ci, for each code block write to new temporary file named something like source-file-md-line-123.yml
  • use original file name then append line number where code block starts
• make pipeline runs for doc changes

regex to find code blocks - working notes:

(?<=```yaml).*?(?=```) - https://regexr.com/

(?s)(?<=```yaml).*?(?=```) - https://regex101.com/

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team