Re-organize Chisel3 Docs section (get rid of the Wiki section) (#114)

I don't think we are getting value of marking the wiki as "Deprecated", as it still contains lots of good content.
Also the Cookbook is not showing up on the website sidebar though it's extremely useful.

* Delete wiki-deprecated.md
* Chisel3: update the landing text
* Introduction: move to Explanation
* Menu: get rid of wiki and move things around
* Add topic headings to the page
* bump chisel3 to test docs reorganization
* chisel3: add an appendix landing screen
* style guide: move to developers section and add a developers landing page
* Introduction: use content from chisel3/docs. Split into introduction and motivation
* Appendix and explanations:  move to chisel3
* Menu: order to match the suggested order and update links to chisel3 content
* Cookbooks: move landing page to chisel3

Co-authored-by: Jack Koenig <koenig@sifive.com>