update to sales handbook

[andre347]

Changes

Added a new handbook page for onsite customer visits covering:

• When to visit customers
• Planning the visit
• During the visit
• Closing off and follow-up

This addresses a gap in the sales handbook around in-person customer engagement.

Checklist

• [x] Words are spelled using American English
• [x] PostHog product names are in title case
• [x] Titles are in sentence case
• [x] Feature names are in sentence case
• [x] Use relative URLs for internal links
• [ ] If I moved a page, I added a redirect in vercel.json (N/A - new page)

[vercel[bot]]

@andre347 is attempting to deploy a commit to the PostHog Team on Vercel.

A member of the Team first needs to authorize it.

[ja-me-sk]

Cool, this can meld a bit with the new page at https://posthog.com/handbook/growth/sales/customer-onsite