docs: Add guide on building custom extensions (e.g. browser plugins)

[vdusek]

Context

In #1921 (and discussion #1920), a third-party stealth-browser integration example was proposed for the "Avoid getting blocked" guide. We decided not to feature fresh third-party tools directly in our docs, since a docs example is effectively an endorsement and we want to keep the bar at proven maturity and adoption.

Internal discussion: https://apify.slack.com/archives/C06ENBV1B9Q/p1779877268101609

Proposal

• Add a docs page describing how to build custom extensions for Crawlee — e.g. subclassing PlaywrightBrowserPlugin to integrate alternative browser backends, custom HTTP clients, etc.
• Third-party projects can then host their own integration guides on their side and we can link to selected ones from this page, instead of maintaining tool-specific examples in our docs.