docs: pact nirvana refresh for branchs/envs

[YOU54F]

Slightly simpler version of #189

I've updated the images in step 4, to use mermaid-js as I don't have the original source, and its got PactFlow graphics in it anyway, so might as well change it up for something that is really easy/open accessible to update in the future

I need to update the images on the others slides as well :)

I've tried to provide some clarification on the PactFlow broker to show that it is

1. Commercial
2. Not open source
3. For the purposes of the guide, it supports either Broker type, and one can revert back to a self hosted Open Source Pact Broker once complete. This may offer less friction for those trying to get buy in, within an organisation.

The PactFlow clarifications are not necessarily meant to be part of this PR, and not the final wording (and not agreed with anyone!)

Updated pages start from

https://deploy-preview-275--docs-pact-io.netlify.app/pact_nirvana/step_3

Updated diagrams

[mefellows]

I'm going to have a crack at updating the remaining mermaid diagrams, merging the text with some changes and getting this out!

[mefellows]

Oh, also, should we get this baby up somewhere: https://github.com/YOU54F/path-to-pact-nirvana?