Feature proposal.md: Update Documentation workflow link.

What does this MR do?

The old link redirects to the general documentation workflow page, instead link directly to the new page's A new feature or feature enhancement section.

Since the link to an id tag is very minor, just updating to https://docs.gitlab.com/ee/development/documentation/workflow.html and changing See the Feature Change Documentation Workflow to See the Documentation Workflow would also be fine.

Does this MR meet the acceptance criteria?

No idea.

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

None needed other than sanity checking that the url is valid.