New docs: Startup strategy information/examples

[rnorth]

Improvement for the new mkdocs-based documentation.

Add a section 'Startup Strategies' to docs/features/startup_and_waits.md describing the default and 'one-shot' strategies, with a codeincluded example for the latter.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you believe this is a mistake, please reply to this comment to keep it open. If there isn't one already, a PR to fix or at least reproduce the problem in a test case will always help us get back on track to tackle this.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you believe this is a mistake, please reply to this comment to keep it open. If there isn't one already, a PR to fix or at least reproduce the problem in a test case will always help us get back on track to tackle this.

[stale]

This issue has been automatically closed due to inactivity. We apologise if this is still an active problem for you, and would ask you to re-open the issue if this is the case.

[vcvitaly]

@rnorth Is it still open for contribution? I'd like to take that up

[bsideup]

@vcvitaly sure! 👍