Fix missing JEPs, move kernel-subshells to follow convention

[krassowski]

Also fixes syntax error in Kernel Handshaking pattern link on the website and moves Notebook v7 to "Implemented' in ToC (it was marked as Implemented in README in my previous #124).

Before	After

Before	After

Changes in the table shading and search bar are due to newer version of sphinx/sphinx-book-theme.

[krassowski]

I don't know if SSC at some point decided not to add numerical prefixes any more and not to update the website, but given that almost none of the JEPs accepted in last two years are discoverable via https://jupyter.org/enhancement-proposals/ which is the public facing index of JEPs, I think this was not a good idea.

[krassowski]

Probably a number of JEPs which I added to ToC/READMEas "Accepted" are already implemented, but I think this PR is already large and I do not want to hold it for discussions on whether something is implemented or not (like subshells are partially implemented).

Also capitalization probably should be made consistent but I mostly kept it as-is just fixing the code syntax as needed.

[krassowski]

CC @jupyter/software-steering-council please review.

[krassowski]

Thanks for the approval! I do not have merge rights here, so someone from SSC or EC will need to do that.

[Zsailer]

Thanks @krassowski! And thank you @minrk for the review. Merging.