Docs: document clamd protocol and clamdscan behavior

Add a dedicated ClamD protocol guide covering socket choices,
command framing, path scans, streaming, file descriptor passing,
reply handling, IDSESSION usage, protocol limits, and tested
examples for the more complex commands.

Link the new guide from the Usage scanning chapter and the mdBook
summary so protocol details have a focused home instead of living in
a shallow command list.

Revise the ClamDScan scanning guidance to describe behavior by
Unix/local and TCP socket type. The user-facing text now avoids
assuming protocol knowledge and avoids presenting tolerated mixed
options as recommended usage.

Author: val-ms

src/SUMMARY.md

@@ -12,6 +12,7 @@
     - [Configuration](./manual/Usage/Configuration.md)
     - [Updating Signature Databases](./manual/Usage/SignatureManagement.md)
     - [Scanning](./manual/Usage/Scanning.md)
+      - [ClamD Protocol](./manual/Usage/ClamdProtocol.md)
       - [On-Access Scanning](./manual/OnAccess.md)
     - [Running ClamAV Services](./manual/Usage/Services.md)
     - [Report a Bug](./manual/Usage/ReportABug.md)