create 2 .md files for the PoC

• /poc/legacy-apps.md
• /poc/disk-encryption.md

Add two .md files for the website's PoC section.

• Running legacy apps has an open issue on GitLab: #44 (#44), but we haven't worked on it yet.
• Setting up LUKS2 FDE and unlocking it using a passphrase or FIDO2. Disc encryption currently doesn't have an open issue on GitLab.
• Typo correction in /poc/manage-fleet.md