update sentry-testkit documentation on docs.sentry.io

[zivl]

There's a section of Sentry-Testkit in official Sentry Docs pages.

We need to enrich the the very initial readme documentation we have there, as it might be outdated and not informal enough.

The sections we need to add are:

• Integration with Puppeteer
• Network Interception

It may be copied from our docs pages. The purpose of this task is so it will be published there as well.

Sentry Docs repo: https://github.com/getsentry/sentry-docs

[github-actions[bot]]

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