Documentation re-org 2018

Current State
GNU Radio documentation lives in one of the following places:


 * 1) Usage Manual in Doxygen (18 pages that cover various topics across GR)
 * 2) "Components" descriptions in Doxygen (various length blurbs about each category of GNU Radio blocks)
 * 3) Doxygen strings for each block, describing arguments and such (this info lives in the same files as the code for the block)
 * 4) Wiki

Problems

 * 1) When you go to https://gnuradio.org/doc/doxygen/ the organization may be confusing for new people, and several of the drop downs don't have useful info.   The wiki home page attempts to provide guidance.
 * 2) We don't define what type of information goes where.  E.g., if someone new-ish wants to get more info about something (e.g. PMTs), it's not clear whether to use the wiki or usage manual.
 * 3) Some information is duplicated across the wiki and usage manual (e.g. the wiki's BlocksCodingGuide is like a condensed version of several usage manaul sections).

Big Question
Whether we want to use doxygen or the wiki for the non-block-specific documentation

Arguments for using the wiki:


 * 1) We have a wiki, non-veteran people may assume that it is the primary source for how-to's and such (especially because it does contain some of that info, just not the well-refined chunk of it)
 * 2) Wiki has more tools suited for this type of non-block-specific documentation
 * 3) Quicker to make a change, changes immediately show up
 * 4) Searching (through Google or the wiki itself) works a little better than searching for something in doxygen

Arguments for using doxygen:


 * 1) You can click functions/blocks/class names and it links to the corresponding page of the API
 * 2) People using an older version of GNU Radio can get the corresponding older version of the usage manual

Proposed Solution if we want to convert to using the wiki

 * 1) One by one, migrate the information in the usage manual sections and components sections to the wiki, removing any redundancies in the process.  On the home page of the wiki, we will link to pages in the wiki instead of pages in the Doxygen usage manual
 * 2) We can make the doxygen usage manual redirect to the wiki, so people use to using it (or who have bookmarked it) will not be disturbed
 * 3) Rename "GNU Radio Manual and C++ API Reference" to "GNU Radio C++ API Reference" and try to remove links to the old usage manual.  What is left will be simply a list of blocks/functions/classes and not be tutorial in nature.
 * 4) The wiki itself needs a re-org, or at least a minor clean up.  Perhaps categorize pages into documentation/tutorials/news/events/archives
 * 5) Figure out how new people registering for the wiki can actually be granted access quickly.  One idea- add a field to the registration process asking for a quick blurb about how they use GNU Radio, and when someone registers it should email someone who can quickly see that it's not a bot, and verify the registration.