docs: add skills glossary

[EfeDurmaz16]

Summary

• Adds a glossary with working definitions for common Skills Over MCP terms
• Links the glossary from the repository contents table

Fixes #51.

Verification

• git diff --check

[olaservo]

Hi @EfeDurmaz16 ! A few different contributors opened glossary PRs, so I started a combined draft here: #104

Lets comment on the combined one instead (and I will add any contributors as co-authors on the commits).