docs: use app.posthog.com for remaining feature-preview links (partial #13516)

[singhvishalkr]

Problem

Partial fix for #13516.

Issue #13516 reports that a "feature previews" link points to us.posthog.com/settings/user-feature-previews, which redirects EU users through the US region instead of keeping them on EU infra.

The originally-reported location (/docs/discussion) has already been updated, but two other docs pages still have the same broken pattern:

• contents/docs/alerts/index.mdx — Anomaly detection alerts beta callout
• contents/docs/cdp/sources/incoming-webhooks.mdx — Incoming webhook sources beta callout

Grep confirms the established convention: 14 other docs/handbook/teams pages already link to app.posthog.com/settings/user-feature-previews (region-neutral). These two were out of step.

# Before this PR
$ rg -l "us\.posthog\.com/settings/user-feature-previews" contents/
contents/docs/cdp/sources/incoming-webhooks.mdx
contents/docs/alerts/index.mdx

# After this PR
$ rg -l "us\.posthog\.com/settings/user-feature-previews" contents/
(no matches)

$ rg -l "app\.posthog\.com/settings/user-feature-previews" contents/ | wc -l
16

Changes

Two-character change per file, us. → app., nothing else.

Test plan

• rg "us\.posthog\.com/settings/user-feature-previews" contents/ returns zero results after the change.
• The two callout boxes still render the same link text (feature preview / Settings > Feature previews).
• No other content on either page was touched.

Notes

This is a partial fix of #13516 — the wider "us.posthog.com → region-neutral URL" sweep has ~50 other hits across contents/docs/ for paths other than user-feature-previews (session replay settings, project settings, account settings, etc.). Happy to take that on as a follow-up if the maintainers want a sweeping PR; I kept this one minimal and grouped by URL pattern so it's easy to review.