Clean up README (#1114)

* Update README.md

* updated readme assets

* delete deprecated lifeline

* update contributing guide and delete old build guides/notes

* fix typos

* update ui formatting recommendation

* update readmes and contributing guide with build steps and dependencies

* add script to install sdk

Co-authored-by: Lucy Cifferello <12953208+elvece@users.noreply.github.com>

backend/install-sdk.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+set -e
+shopt -s expand_aliases
+
+if [ "$0" != "./build-sdk.sh" ]; then
+	>&2 echo "Must be run from backend directory"
+	exit 1
+fi
+
+cargo install --bin=embassy-sdk --path=. --no-default-features --verbose