doc: update user guide with build instructions and dependency changes

This commit updates the user guide to reflect the use of Ubuntu 22.04 LTS
and includes additional prerequisites such as `libssl-dev` and `telnet`.
It also improves the build instructions by clarifying the use of Clang,
setting `LLVM_SYSROOT_CC` to `arm-none-eabi-gcc`.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>