More asset version details

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no, but follows after #3742 
| Applies to    | 2.5+

Hi guys!

This adds some further tweaks to the new feature and docs from #3742:

* moved the absolute asset URL generation part below the new asset versioning feature in the `book/templating` page so that we talk about the "global asset versioning" (i.e. the config option in config.yml) and this new feature right next to each other.

* Added some additional details to make it clear that there is a "global" asset version, and then the ability to override on an asset by asset basis.

I think it's all small details, but @romainneutron if you have a quick chance to look this over, that would be awesome too.

Thanks!