Docs review and re-organization

This:

* Re-organizes the docs into sections that align with what docs.filecoin.io becoming:
  * An installation section
  * A "getting started" section (lotus client focused)
  * A "storing" section (lotus client focused)
  * A "mining" section (miner focused)
  * A "build" section (developer focused)
  * An legacy "architecture" section is left in the last place.

A few high-value documentation pages have been reviewed and updated with the latest recommendations:

* Installation section and lotus setup
* Miner setup
* etc.
...

Other pages have been correctly merged into the new relevant sections. Some pages have not been touched. The filesystem layout of the documentation has been changed into folders corresponding to the sections (as requested by @cw). Some pages that were not linked at all and/or where hidden, have been moved to "unclassified".

This should make the porting of the Lotus documentation to docs.filecoin.io much easier, while ensuring it is more up to date than it was before.

For the moment, this breaks most links as link-aliasing is not supported in lotus-docs.

Makefile

@@ -280,7 +280,7 @@ method-gen:
 gen: type-gen method-gen
 
 docsgen:
-	go run ./api/docgen > documentation/en/api-methods.md
+	go run ./api/docgen > documentation/en/building/api-methods.md
 
 print-%:
 	@echo $*=$($*)