docs: recommend users install the GitHub or Bitbucket Cloud hosted app

[HonkingGoose]

Changes

• Add short intro sentence to GitHub and Bitbucket Cloud platform module pages
• Explain that the hosted app is the easiest way to get started

Context

Closes #29372.

We should point out the easiest way to do things first.

Some users may land on the platform module docs via the ## Supported platform section of the docs homepage, which directly links to the platform module docs:

## Supported Platforms

Renovate works on these platforms:

- [GitHub (.com and Enterprise Server)](./modules/platform/github/index.md)
- [GitLab (.com and CE/EE)](./modules/platform/gitlab/index.md)
- [Bitbucket Cloud](./modules/platform/bitbucket/index.md)
- [Bitbucket Server](./modules/platform/bitbucket-server/index.md)
- [Azure DevOps](./modules/platform/azure/index.md)
- [AWS CodeCommit](./modules/platform/codecommit/index.md)
- [Gitea and Forgejo](./modules/platform/gitea/index.md)
- [Gerrit (experimental)](./modules/platform/gerrit/index.md)

If users follow a link like this, and they end up at a platform which has a hosted app, we should:

• point out the easiest way first
• Explain that the info in this docs is mostly meant for self-hosted users. (The issues list for the platform can be relevant to both hosted and self-hosted users).

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 37.413.5 🎉

The release is available on:

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

Your semantic-release bot 📦🚀