use id to find the future current sidebar item (#586)
use id to find the future current sidebar item (#586)

Fixes #583

- find the referenced 'id element' first, then the closest section
- if no association found in the sidebar, fallback to #
- reword comments
snide/sphinx_rtd_theme,snide/sphinx_rtd_theme,snide/sphinx_rtd_theme,snide/sphinx_rtd_theme