v8 API documentation

[martincostello]

As part of the release of Polly v8 we should ensure that either before or soon after release we have reviewed all of the existing documentation to ensure that we have:

• Updated the README and wiki to cover the v8 API
• Add any relevant new documentation that isn't covered by the above (e.g. brand new features)
• Have a migration guide for moving from v7 to v8
• Have a guide on how to wrap the v7 API with with the v8 API (i.e. a partial migration)

[github-actions]

This issue is stale because it has been open for 60 days with no activity. It will be automatically closed in 14 days if no further updates are made.

[martintmk]

Let's mention the order of executions when adding strategies which is changed compared to Wrap in V7:

#1365 (comment)

[martintmk]

Let's also make our code-snipets future-proof by injecting them into markdowns:

#1539 (comment)

cc @martincostello