does the doc folder contain only documentation?

It's specified by the TeX Directory Structure (TDS):

3 Top-level directories

• ...
• doc for user documentation

So, if there would be anything else than documentation, it would be in the wrong place.

Though I consider documentation as very important, I understand that you just change the way of access to online. Just note, that online documentation might not match the versions you have locally installed.

A good source for you: TeXdoc.net, based on TeX Live 2012 and regularly updated. It even offers a "quick search" you could use in the search field at the top right of the Firefox browser.

Please interpret this answer as having the best possible intention of helping you. :)

Although this doesn't tell you how to do what you've asked for, and although I don't have any idea about your personal "disk space circumstances", I'd strongly recommend pursuing a different solution.

One option is simply "get more disk space"; I accept this may not be an option in your case.

For another approach, you may care to try a tool like WinDirStat (for Windows -- I have no experience of doing this on Linux, though KDE has KDirStat, I think). This will allow you to find the "hot spots" quickly.

Experience suggests that micro-managing space by trimming lots of small files doesn't usually give you a good return for your time, when compared with offloading, say, three films onto an external drive.

My current TeXLive texmf-dist/doc subtree is 1.25 Gb; I think it's worth every byte to have it at my fingertips.

The post is old, but for actual readers: as far I know, any package update will reinstall them, so deactivate this in your manager. For example "tlmgr option docfiles 0", or option it out in preferences of TexLive Utility.