Pipes section needs more introduction material

[chrillek]

Currently, the only explanation is
Hugo Pipes is Hugo’s asset processing set of functions.
It would be helpful to mention what an "asset" could be, and also that they are located under the assets directory. The latter might be self-evident for people already familiar with Hugo. But not for newbies.

Something along the lines of

Pipes allow you to process files under the `assets` folder in various ways. For example
- transform SASS files into CSS
- Bundle and transform JavaScript files
- …
These files are accessed through the `Resource` object, which can refer to local or remote files.

Such an introduction would make it (I think) clearer why pipes are relevant and what they can be used for, thus enticing people to continue learning about them.

[jmooring]

This entire section needs to be reworked, pointing to the relevant method pages instead of duplicating the examples. I'm not even sure it needs to be its own section, but that needs some thought.