docs: [#414] post-deployment manual steps for floating IPs and IPv6

[josecelano]

Overview

Closes #414

Adds a new provider guide documenting the manual post-deployment steps required when using Hetzner floating IPs and/or IPv6 UDP tracker support. Also reorganizes provider documentation into per-provider subdirectories.

Changes

• New: docs/user-guide/providers/hetzner/post-deployment.md — step-by-step guide covering:
  • Step 1: Floating IP policy routing (netplan configuration for symmetric routing)
  • Step 2: Enable Docker ip6tables management
  • Step 3: Enable IPv6 on the Docker bridge network
  • Step 4: SNAT for IPv6 UDP replies via floating IP
  • Scenario matrix showing which steps apply to which setup
• Reorganized: provider docs moved into per-provider subdirectories:
  • docs/user-guide/providers/hetzner.md → docs/user-guide/providers/hetzner/README.md
  • docs/user-guide/providers/lxd.md → docs/user-guide/providers/lxd/README.md
• Updated: docs/user-guide/providers/README.md — updated links to new structure
• Updated: all files referencing the old provider doc paths (13 files)
• Updated: project-words.txt — added SNAT and POSTROUTING to the project dictionary

References

• Issue: docs: post-deployment manual steps for floating IPs and IPv6 #414
• Reference implementation: torrust/torrust-tracker-demo
• Root-cause investigation: torrust-tracker-demo#2

[josecelano]

ACK 2dda404