Add unique IDs to section headings in Aliki template (#1603)

The Aliki TOC is generated by JavaScript that scans h1/h2/h3 elements
with IDs. Section title h2 elements had no ID, so they were invisible to
the TOC. Meanwhile, h3 IDs like "constants" and
"public-instance-methods" were duplicated across sections.

Prefix all heading IDs with the section's aref to make them unique. Move
the section ID from the `<section>` element to the `<h2>` so the TOC
picks it up naturally.

<img width="250" height="291" alt="Screenshot 2026-02-08 at 10 20 30"
src="https://github.com/user-attachments/assets/b399fb90-ff1a-46b9-838c-61576c6acc2c"
/>


Fixes #1598

Author: st0012