docs: Fixes to N Preview API overview

Many HTML problems (bad links, non-escaped < and &, unclosed blocks);
fixing the ones I saw. We should give this a more thorough scrubbing.

Also removed spurious link to (non-existent) "Testing with Just in
Time" section.

See first comment for doc stage location.

Change-Id: I07ca445bc73aee012e02ff3ce9765efcde9df14d