T6342: extend schema to handle documentation in xml

[rebortg]

Change summary

This PR adds support for structured documentation in interface definitions by extending the RELAX NG schema. The new docs element allows for better organized documentation with the following optional elements

<properties>
  <help>Container applications</help>
  <priority>450</priority>
  <docs>
    <headline>Containers</headline>
    <text>The VyOS container implementation is based on https://podman.io as a deamonless container engine.</text>
    <usageExample>
      set container name mysql-server image mysql:8.0
      set container name mysql-server image quay.io/mysql:8.0
    </usageExample>
    <hints type="danger">If a registry is not specified, Docker.io will be used...</hints>
  </docs>
</properties>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T6342

Related PR(s)

vyos/vyos1x-config#32

How to test / Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[github-actions]

CI integration ❌ failed!

Details

CI logs

• CLI Smoketests (no interfaces) ❌ failed
• CLI Smoketests (interfaces only) ❌ failed
• Config tests 👍 passed
• RAID1 tests 👍 passed
• TPM tests 👍 passed

[c-po]

Could you please clean the stray whitespaces?