Update docs to include Developer Mode as prereq

[sirredbeard]

Summary of the Pull Request

Adds Developer Mode to prereqs for building WSL from source.

PR Checklist

• **Closes: Building WSL from source requires Developer Mode #12923
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Minor docs addition to advise developers to enable Developer Mode to build WSL from source.

Validation Steps Performed

1. Attempted to build WSL from source, but failed, see error in Building WSL from source requires Developer Mode #12923.
2. Enabled Developer Mode.
3. Build successfully completed.

[OneBlue]

Thank you for doing this ! Writing this comment to see if that will cause the VSO pipeline to start :)

[OneBlue]

/azp run wsl-github-pr

[azure-pipelines]

No pipelines are associated with this pull request.

[OneBlue]

/azp run

[azure-pipelines]

No pipelines are associated with this pull request.

[OneBlue]

LGTM ! This isn't directly wired to wsl.dev so this won't apply immediately, but it will get updated once we connect the domain to that repo.

Thank you for doing this !