docs: add network hosting and reverse proxy troubleshooting (@isma-021)

[isma-021]

Description

I noticed that when trying to host Monkeytype over a local network or the internet instead of localhost, the frontend crashes with an Uncaught TypeError: crypto.randomUUID is not a function error. This happens because modern browsers restrict this API to secure contexts (HTTPS).

When putting a reverse proxy in front to handle HTTPS, it's very easy to run into Mixed Content blocks or 404 errors due to incorrect .env configurations (like adding a trailing slash to MONKEYTYPE_FRONTENDURL or forgetting to completely recreate the containers).

This PR updates SELF_HOSTING.md to:

• Explain the HTTPS requirement for network deployments.
• Detail how to properly configure the backend URL in the .env file to avoid API 404 errors (the trailing slash issue).
• Highlight the importance of using docker compose up -d --force-recreate and clearing the browser cache, since environment variables are baked into the SPA static files at startup.

Checks

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[Miodec]

Thanks!