feat:On-page table of contents for read-only docs (collection, folder, request)

[sachin-bruno]

Ref: BRU-4087

Problem
Rendered API-docs pages get long: a single request stacks Params, Body, Headers, Auth, Execution Context (with its own tabs), a code snippet, and free-form markdown, while overview and folder pages add arbitrarily long documentation on top. There was no way to see a page's structure at a glance, jump straight to a section, or tell where you are while scrolling — readers had to scroll and hunt, and the docs lacked the "on this page" outline that almost every documentation site provides.

Fix
Adds a SectionNav "on this page" navigator: a fixed right-edge rail of tick marks (a mini-map of the page) that expands on hover/focus into a labelled table-of-contents popup, with scroll-spy that highlights the section you're currently reading. Pages declare their own outline through DOM markers (data-nav-section / data-nav-headings), so the navigator stays in sync with whatever each page renders — including markdown h1–h6 as nested entries — rather than hardcoding a structure per page.

[vasharma05-bruno]

LGTM!