docs: update warden_server build docs for submodule + FetchContent

- ciphers.md: rewrite "Building libwarden_server from source" — warden_server is now a git submodule (PrimalConquer/warden_srv @ v6600), not an in-tree subtree

- ciphers.md: libsodium now via CMake FetchContent (pinned libsodium-cmake) instead of a nested external/libsodium submodule; note network needed on first configure

- ciphers.md: switch build steps to the CMake presets with -DCMAKE_POLICY_VERSION_MINIMUM=3.5, fix output paths (build//bin/), and replace the obsolete --recursive gotcha

- in_progress.md: note the subtree->submodule conversion and committed warden_server.dll / libwarden_server.so under the Warden server P/Invoke migration section