docs: add a self-hosted server option under Self-host

[vikasprogrammer]

I put this under Self-host rather than next to the one-click buttons at the top, because that is where it actually belongs. Your Self-host section already says to use nginx or Caddy with a TLS certificate for a real server, and this is a way to get exactly that without setting one up: it clones the repo onto a Linux box and serves it over HTTPS.

I deployed it as it is to check first. Here it is running: https://b2-2fa.nbg1-2.instapods.app

To be straight with you about where this helps and where it does not. For most people Pages or Cloudflare is free and fine, and your README is right to lead with those. The case for a server is narrower: someone who would rather their TOTP page did not sit behind a third-party edge network, on their own domain and their own box. If you think that reader is too rare to be worth a line in the README, close this, no hard feelings.

Disclosure: I work on InstaPods, so I am not a neutral party here.

[vikasprogrammer]

Withdrawing this one myself, no action needed from you. We were trying out README deploy links across a few projects and have decided to stop, so I would rather close it than leave it sitting in your queue. The demo instance in the description is being taken down too, so that link will stop working shortly. Thanks for the project, and sorry for the noise.