Add Mend app guide to the top of Bitbucket Cloud readme

[rarkins]

Describe the proposed change(s).

The easiest way to get started on Bitbucket Cloud is now using the Mend app with free Renovate plan. But if you search for something like "how to get started with Renovate for Bitbucket Cloud" you find https://docs.renovatebot.com/modules/platform/bitbucket/ without any mention of it.

Ideally docs.mend.io should have a perfect landing page for how to use the free Renovate. The only current docs page there (link) is too focused on Mend's paid options and can confuse those who want a free plan. We should add a page there for how to get started with Renovate in Mend, but in the meantime we can have docs on docs.renovatebot.com instead.

Regardless, these docs should be the first thing you read at https://docs.renovatebot.com/modules/platform/bitbucket/, which is built from https://github.com/renovatebot/renovate/blob/main/lib/modules/platform/bitbucket/readme.md

[rarkins]

@justo-mend this is for you

[renovate-release]

🎉 This issue has been resolved in version 37.413.5 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 37.413.5

Your semantic-release bot 📦🚀