improve dynamic os installation instructions

Nobody has claimed this yet.

Step 3 assumes os is installed but os is installed in step 4

as [discussed in matrix](https://matrix.to/#/%23OSFW-Heads%3Amatrix.org/%24LUP5Eh2OS9kBTth1SbqY_bgInCId4sDOTHxtAz60tsQ?via=matrix.org&via=nitro.chat&via=tchncs.de) step 3 maybe can be split into separate categories like step 1 for example "no os installed" and "os installed" and maybe also another one for "reinstalled os"

noteable excerpts from matrix:
user1:
Not sure I see the inconsistency

Users are supposed to land at https://osresearch.net/Install-and-Configure
```
Step 1 - Building Heads
Step 1 - Downloading Heads
Step 2 - Flashing Guides
Step 3 - Configuring-Keys
Step 4 - Installing Qubes and other OSes
```

Where users can either install OS first (recommended if you flashed) when then Heads will pick on

Logic under Heads is
- No OS installed: guide user into USB booting and installing.
- OS dectected installed but no public key fused in firmware: propose user to inject public key or do OEM Factory Reset/Re-Ownership

Then follow. The problem lies when users reinstall OS: if there is public key fused, check passes. But then OS /boot has not digest, so probing

users to sign, but no way to verify, currently, that fused key is the one matching private key counterpart.

User should make sure public key fused is good or do re-ownership if they reinstall.

Most users will go through flashing+installing OS + configuring keys + configuring default boot + TPM DUK.

Problems lies when steps are not followed, or OS reinstall.

Start at the Install-and-Configure page linked in the issue and review the current Step 3 and Step 4 instructions alongside the Matrix discussion. Clarify the paths for no OS installed, an existing OS, and an OS reinstall, including the stated public-key and re-ownership considerations. Done means the sequence no longer assumes an OS is installed before Step 4 and covers the cases described.

Written by the indexing model from the issue text.