Fix broken internal links in concept files

## Description

Several concept files have broken internal links that need to be fixed:

### Files with broken links:

1. **concepts/progressive-trust.md** (line 457)
- Current: `[XID Fundamentals](xid-fundamentals.md)`
- Should be: `[XID Fundamentals](xid.md)`

2. **concepts/public-participation-profile-examples.md** (lines 17, 409)
- Line 17: `progressive-trust-lifecycle.md` → should be `progressive-trust.md`
- Line 409: `progressive-trust-lifecycle.md` → should be `progressive-trust.md`
- Line 409: `xid-fundamentals.md` → should be `xid.md`

### Note
The `public-participation-profile-examples.md` file also has a warning at the top indicating it hasn't been fully checked and proofed, so it may need additional review beyond just fixing the broken links.

### Related PRs
This follows up on the documentation improvements made in:
- PR #9: Improved concepts/README.md
- PR #10: Synced main README.md with concepts improvements
- PR #11: Fixed broken links in LEARNING_PATH.md