Add HaProxy conf with/without TLS termination

[paolo-lulli]

Pull Request Checklist

The PR documents how to use HaProxy as reverse proxy towards Dendrite in the following 2 scenarios:

• with TLS termination
• without TLS termination

• [x ] I have added Go unit tests or Complement integration tests for this PR or I have justified why this PR doesn't need tests
• [x ] Pull request includes a sign off below or I have already signed off privately

Signed-off-by: Paolo Lulli <paolo@lulli.net>

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