docs(guides): several fixes (links, typos, syntax, line lengths)

I started out fixing line-length to comply to the max. 100 chars per line rule (as discussed [here][1]), but this turned out to be a more generic error fixing:

* Fixes wrong or broken links
* Fixed typos/punctuation/syntactic errors
* Rephrazed at a couple of places, where the wording sounded unclear or confusing 
* Wrapped lines at 100 characters

<sub>The diff may seem a little weird due to the line-wrapping.</sub>

--
I have re-read all the modified documents and tried all the links (and everything seems fine), but a 2nd pair of eyes is always irreplaceable :smiley: 

[1]: https://github.com/angular/material/pull/819#issuecomment-65216793
The reason side-by-side review is difficult is because I **have** linebreaks, but the old version didn't. Basically, the original intent of this PR was to apply the max. 100 chars/line rule the docs, because the long lines made reviews difficult. So, hopefully, future reviews will be much easier :)