[Feature]: add Zenith Hosting as a row in the README installation methods table

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Summary

hey, i work on Zenith Hosting, a startup doing one-click hosting for open source apps. we added Appsmith to our marketplace, so i would like to add one row to the Installation Methods table in `README.md`, alongside the existing Docker / Kubernetes / AWS AMI rows, linking to https://zenith.hosting/host/appsmith.

it would be a single line, additions only, using a height-matched badge so it sits level with the existing `for-the-badge` shields. it is meant as one more option in that list, not as a replacement for any of them, and it does not touch the Appsmith Cloud line above the table.

raising this first per the contribution guidelines. PR #42078 is already open as a draft with the exact diff if it is easier to look at that.

### Why should this be worked on?

people who want self-hosted Appsmith rather than Appsmith Cloud currently have to run and maintain the server themselves, which is where a chunk of them stall. the table is the place they look for that, so listing a managed option there is one less dead end for the non-devops end of your self-host audience.

happy to reword it, move it, or close this if you would rather not link third-party hosts in the README. we also share a cut of revenue with the projects we host, happy to explain if you're curious: hello@zenith.hosting

Review README.md's Installation Methods table and compare the proposed placement and badge with the neighboring Docker, Kubernetes, and AWS AMI rows. PR #42078 contains the exact diff; done means the agreed Zenith Hosting row is present without changing the Appsmith Cloud line or existing options.

Written by the indexing model from the issue text.