Another documentation update

- Wrote "Getting Started" page which brings together content previously in
  multiple places.

- Moved build system, porting, and trace documentation to
  docs/development/.

- Moved networking documentation to its own subdirectory of features/.

- The toplevel index.rst no longer contains the toplevel table of contents;
  instead the table of contents is separate.  This helps make the chapters
  in the generated PDF come out sanely.

- Removed toplevel RELEASE file as it's out of date and will need to be
  re-written for the next release anyway (or turned into an incremental
  NEWS file).