Fix broken YC Deal page links in handbook (#17150)

* Fix broken YC Deal page links in handbook

The YC deal/onboarding content was moved out of the handbook and into the PostHog app (app.posthog.com/startups/yc). A redirect exists for the short path /yc-onboarding but not for the old full handbook path /handbook/growth/sales/yc-onboarding, so links pointing at the old path 404.

Repoint all three references to /yc-onboarding.

Generated-By: PostHog Code
Task-Id: 96cc1f6b-2465-4ad1-b2f2-691381242a31

* Point YC links to startups handbook page instead of gated app page

/yc-onboarding redirects to the actual app onboarding page, which is gated to teams on those plans with the right access. Point the handbook references to the public Startups & Y Combinator handbook page (/handbook/brand/startups) instead.

Generated-By: PostHog Code
Task-Id: 96cc1f6b-2465-4ad1-b2f2-691381242a31

* Adopt reviewer suggestions: restore YC link text

joethreepwood's review suggestions retargeted the YC links to /handbook/brand/startups while keeping the original descriptive link text ("YC Deal page", "YC Onboarding"). A later commit applied the same URL but rewrote the text to "Startups & Y Combinator", which marked the human suggestions outdated. Restore the reviewer's wording.

Generated-By: PostHog Code
Task-Id: 39c38366-d9c6-4a1b-8e7a-11d924b252a5

---------

Co-authored-by: Joe Martin <84011561+joethreepwood@users.noreply.github.com>

Author: pauldambra

contents/handbook/company/merch-store.md

@@ -57,7 +57,7 @@ As always, we expect you to use this with restraint and with your own good judge
 
 ### YC Deal
 
-You can find instructions for this on the dedicated [YC Deal page](/handbook/growth/sales/yc-onboarding).
+You can find instructions for this on the dedicated [YC Deal page](/handbook/brand/startups).
 
 ## Troubleshooting customer orders
 

contents/handbook/growth/sales/automations.md

@@ -244,7 +244,7 @@ Remember to update the Subscription in the Billing Admin Portal
 
 ## YC Program
 
-This process is documented in the [YC Onboarding](/handbook/growth/sales/yc-onboarding) section of the handbook.
+This process is documented on the [YC Onboarding](/handbook/brand/startups) page.
 
 ## PostHog for Startups
 

contents/handbook/people/onboarding.md

@@ -155,7 +155,7 @@ We use a number of different tools to organise our work and communicate at PostH
 -   Fondo - US accounting
 -   Deel - international payroll and contracts management
 -   Ashby - recruitment
--   Micromerch - merch inventory management, [YC onboarding](/handbook/growth/sales/yc-onboarding) merch, and merch [drop-shipping for small events](https://github.com/PostHog/meta/issues/31)
+-   Micromerch - merch inventory management, [YC onboarding](/handbook/brand/startups) merch, and merch [drop-shipping for small events](https://github.com/PostHog/meta/issues/31)
 
 ## Signatories