index.md: Proofreading/formatting pass

As a beginner with the project, I read this doc and tried to address issues
I spotted as I go. This includes:

* Typos
* Improvements to Markdown formatting.
* Better wording in some places.
* Removed dangling duplicate ToC entries for the previously removed
  duplicate section.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ic619f1f4703c4b629448343d1c4b2e0affa6b206
1 file changed
tree: 697944d58c36ed17cc9318d5175a77f393285fc4
  1. docs/
  2. .gitreview
  3. mkdocs.yml