docs: change /microagents/README.md to /skills/README.md

[OTR]

The documentation referenced an outdated path. Microagents have been renamed to skills in OpenHands terminology, so the documentation should reflect the correct path to the skills architecture documentation.

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

[enyst]

Thank you!