Custom domain setup instructions and React example are outdated

[simple-analytics-ai]

What

The custom-domain guide still describes the old SSL checker and omits the DNS validation and certificate behavior introduced in dashboard PR #843. The React guide also loses the ignoreMetrics values when Jekyll renders its JSX example.

Why

Readers can follow obsolete setup instructions or copy an incomplete example.

Notes

Documentation review: https://github.com/simpleanalytics/dashboard/pull/843#issuecomment-5621574516