minor improvements to Debian/Ubuntu based install guide

[dch]

Without erlang-dev and erlang-xmerl, it's not possible to build mint, nor phoenix.
It seems reasonable to add these as general dependencies for everybody.

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 22.04.3 LTS
Release:        22.04
Codename:       jammy

$ mix hex.info

Hex:    2.0.6
Elixir: 1.15.7
OTP:    26.1.2

Built with: Elixir 1.14.2 and OTP 23.3

> Compiling 1 file (.erl)
    src/mint_shims.erl:37:14: can't find include lib "public_key/include/public_key.hrl"
    %   37| -include_lib("public_key/include/public_key.hrl").

[dch]

see phoenixframework/phoenix#5552 for another person running into this.

[josevalim]

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