[Request]: Document Private Certificates support for Synthetics browser monitors

[vigneshshanmugam]

Description

The previous recommended way to use Private Certs was always to use a custom docker image with certs mounted locally and updated to trust the chromium certificates store. This no longer holds true with the recent introduction of [playwrightOptions.clientCertificates](https://github.com/elastic/synthetics/pull/952) option where users can configure in-memory certificates or use certificates on the local path when running browser monitors.

Resources

elastic/synthetics#952

Which documentation set does this change impact?

Stateful and Serverless

Feature differences

Identical.

What release is this request related to?

8.16

Collaboration model

The engineering team

Point of contact.

Main contact: @emilioalvap

Stakeholders: @vigneshshanmugam @bturquet @drewpost