Fix broken link in hosted-quickstart.mdx

[arpagon]

The link to the Hosted Walkthrough in docs/hosted-quickstart.mdx is broken. It currently points to '/getting-number' but should point to '/walkthrough_intro' or './walkthrough_intro'.

Current: Hosted Walkthrough

Should be: Hosted Walkthrough

or

Hosted Walkthrough

File to update: docs/hosted-quickstart.mdx