Yii3 guide links don't work

[mspirkov]

The links don't open because there are no guides. We need to generate guides, as we do for Yii1 and Yii2.

[samdark]

Currently the guide is generated differently: https://yiisoft.github.io/docs/guide/. I think that's the case for many projects where the guide is done in a unique way. I think it's a good idea to add a "guide base urls" option for apidoc i.e. guide: should prefix everything with https://yiisoft.github.io/docs/guide/, cookbook: — with https://yiisoft.github.io/docs/cookbook/ etc.