[DOC] Tidy up `index`, `governance` and `code_of_conduct` pages.

[MatthewMiddlehurst]

For the most part both the code of conduct and governance remain the same, it is just minor language changes. The exception to this is an alteration to

All non-sensitive project management discussion takes place on the contributors channel on the project Slack and/or the issue tracker. Occasionally, sensitive discussion and votes such as appointments will occur in private Slack channels.

as this has turned out to not really match how we develop since the time the document was created. The changes here could be seen as an alteration of expected practice.

The index page removes the big v1.0.0 warning, mention of forecasting (for now), and adds experimental modules.

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