Convert mediacapture-fromelement to Bikeshed

[yellowdoge]

I propose moving this Spec to bikeshed in the spirit of w3c/mediacapture-image#139 and w3c/mediacapture-record#106. Both of those issues are closed and the Specs shifted to Bikeshed which IMHO is must more succinct and cross-ref friendly.

I already migrated the document (see the result here); in the process I corrected a few dead/wrong links. The Table of Contents is on ToT:

table of contents
1. Introduction
2. Conformance
3. HTML Media Element Media Capture Extensions
A. References
A.1 Normative references
A.2 Informative references

and I corrected it to:

Introduction
2 HTML Media Element Media Capture Extensions
 2.1 Methods
3 HTML Canvas Element Media Capture Extensions
 3.1 Methods
 3.2 CanvasCaptureMediaStreamTrack
  3.2.1 Attributes
  3.2.2 Methods
4 Security considerations
5 Acknowledgements

;Like in the mentioned Specs,:

@dontcallmedom and I had a small offline conversation and decided to migrate to .bs directly on the gh-pages branch and upload the compiled corresponding html file together, not using any Travis magic. If we see that this is not cool, we can revisit some Travisification of the process.

[yellowdoge]

ping @dontcallmedom , can you help us to automatize bikeshed-compile-and-commit-to-gh-pages ?