Add Zenith to the One-Click Deploys list in the README

Describe the improvement you are suggesting in detail

hey, i work on Zenith Hosting, a startup doing one-click hosting for open source apps. we added Documenso to our marketplace, so people can run a managed instance without touching a server.

i'd normally open a small PR for this, but CONTRIBUTING says external PRs aren't being merged right now, so an issue it is. happy to send a patch if you'd rather have one, or you can just paste the two lines below.

the suggestion: add a Zenith entry to the "One-Click Deploys" list in the README, alongside Railway, Render, Koyeb and Elestio.

#### Zenith

[![Deploy with Zenith](https://cdn.zenith.hosting/buttons/deploy-with-zenith.svg)](https://zenith.hosting/host/documenso)

one-click managed Documenso: storage, backups, email and a free subdomain included. no referral or tracking parameters on the link.

we share a cut of revenue with the projects we host, so this would send something back to Documenso rather than just being a listing. happy to explain how that works: [email protected]

completely your call, and no hard feelings if the answer is no. mostly wanted to check whether you'd take it before doing anything.

Additional Information & Alternatives (optional)

• the README already carries four third-party hosting buttons, so this is the same shape of change, one heading and one badge line in the "One-Click Deploys" section.
• alternative placement: the self-hosting docs at docs.documenso.com rather than the README, if you'd prefer to keep the README list short.
• the badge image and link both resolve today: https://zenith.hosting/host/documenso
• nothing here needs code changes or affects the license.

Please check the boxes that apply to this improvement suggestion.

• I have searched the existing issues and improvement suggestions to avoid duplication.
• I have provided a clear description of the improvement being suggested.
• I have explained the rationale behind this improvement.
• I have included any relevant technical details or design suggestions.
• I understand that this is a suggestion and that there is no guarantee of implementation.