feat: add opensearch support

[qwerzl]

This PR includes two parts:

1. Detection for url param q. If detected, open search page automatically.
2. Add opensearch.xml generation. The link is added to head of each page. The url is configured via opensearch.url in AppConfig.

[Atinux]

Thank you!

I would be nice to document it, also I think we could move the url to the docus.url part, wdyt?

[qwerzl]

Thank you!

I would be nice to document it, also I think we could move the url to the docus.url part, wdyt?

I've changed the url.

However I think as docus.url is not a new config key, there's no need for us to update docs?

[Atinux]

Thank you!