docs: persistent subscription

[st0nie]

Background

Some times it may not be possible to pull the subscription when the network is poor, which can lead to users not being able to function properly if they are completely dependent on the subscription.(no node exists in the group)

Add documentation for persistent subscriptions to help users avoid this issue.

Checklist

• The Pull Request has been fully tested
• There's an entry in the CHANGELOGS
• There is a user-facing docs PR against https://github.com/daeuniverse/dae

Full Changelogs

• [Implement ...]

Issue Reference

Closes #[issue number]

Test Result

[mzz2017]

That's pretty cool. I think it will be useful to a lot of people. Thanks for your contribution, let's help make dae better together.

[umlka]

建议增加一个UA

[douglarek]

Frankly speaking, this thing should be implemented in the code rather than being mixed up in the documentation. For example, add retries and scheduled updates.

[mzz2017]

Thanks for your contribution!