chore(docs): initial restructuring

[chdeskur]

@coderabbitai summary This PR initializes the structure for larger SDK docs upgrades. Noted additions:

• Overview page
• Language configuration page
• Moving of the reference/guides section

Has been checked for broken links as a result of the restructuring 🟢

[github-actions[bot]]

🌿 Preview your docs: https://fern-preview-ea90826b-a595-40f9-b2b8-63e2de14ee5c.docs.buildwithfern.com/learn