feat(org): anchor back-links to project sections in index

feat(org): anchor back-links to project sections in index

Update every project's "Back to physical projects" link to include a
fragment that scrolls the index page to the TOC section where the
project is listed. Previously every back-link pointed at the top of
the index, dropping users at the Overview section.

The anchor is derived from the parent section's CUSTOM_ID (e.g.
#outline-container-optics for top-level Optics, #outline-container-
measurement-device-cases for cases under that sub-heading). Projects
that aren't listed in the index fall back to the top-level category
they live under.