docs refresh without OS pages

[jtmoree-github-com]

This is the end result of https://github.com/osresearch/heads-wiki/pull/66#issuecomment-832656886 without the OS specific pages which were the most difficult to work out. I created this branch to clean up commit history and remove the controversial pages.

• move content from index to prerequisites page
• make index a generic overview
• add recovery shell page
• fix note about security dongle in About/Keys
• move boot options from os install to separate page
• link to recovery shell page in upgrading
• minor updates to upgrading about disk recovery key

[tlaurion]

@jtmoree-github-com Will review!! Thanks for the massive cleanup!

Is there an online version that can be consulted, following instuctions at https://osresearch.net/Contributing-to-Heads-wiki/#testing-changes-on-github ?

[jtmoree-github-com]

@tlaurion https://jtmoree-github-com.github.io/heads-wiki/ has the changes for this PR.

[tlaurion]

@jtmoree-github-com reviewed!

[jtmoree-github-com]

@tlaurion I believe we have addressed all of your concerns. Please merge this PR.

[tlaurion]

@jtmoree-github-com thanks a bunch!