docs: add usage instructions and self-hosting guide

[ggfevans]

Summary

• Add "How to Use" section to main README with a 5-step guide for new users
• Add description highlighting privacy features (no accounts, no backend, client-side encryption)
• Add comprehensive self-hosting guide (README-SELFHOST.md) covering:
  • Quick start with Yarn and npm options
  • Base path configuration for different hosting setups
  • Deployment examples for Nginx, Caddy, and Docker
  • Development setup instructions
  • How the encryption and privacy features work
  • Customisation options for background and colours
  • resolves Add how to deploy it to the readme #42

Test plan

• Verified all links work correctly
• Tested build instructions with both yarn and npm
• Reviewed for consistency with existing project style

[n-b]

Merci pour la doc! Sur apache, j’ai du rajouter cette ligne (à la place du try_files de nginx ou caddy):

<Directory /var/www/secret-santa/dist/>
  RewriteEngine on
  RewriteRule ^pairing$ index.html
</Directory>

[ggfevans]

Je dois pratiquer mon français ! J'avais oublié que c'était Apache et pas nginx — nginx c'est mon défaut. Merci encore pour cette solution super simple ! Dis-moi si tu veux que je fasse des changes au PR pour faciliter l'implémentation.

[n-b]

Ah, sorry for the confusion! I’m not the maintainer of that projet, I actually just found out about it this morning while looking for a self hosted secret santa tool, had trouble with the configuration, and found the solution in that PR. I also didn’t even realise I was responding in french to a message in english 😬.

To be honest, I don’t know how the url rewriting for /pairing works at all on github pages. I just added the apache conf that worked for me — and I’m by no means an expert.

[ggfevans]

Hah, I replied late at night and somehow confused you with the repo owner. I'm also no expert!