docs: fix software support matrix overview link and sidebar context

[tzz-radxa]

What

Fixes two issues on the published Software Support Matrix pages:

1. Overview page shows no sub-navigation and the link is broken

   • The overview (/software-support-matrix) previously set displayed_sidebar: home, so it stayed on the global home sidebar instead of switching to the matrix's own autogenerated sidebar (where the Rockchip sub-page lives). Other top-level sections (e.g. compliance) don't set displayed_sidebar, so their overview page shows the section sidebar with children. Removed it.
   • The in-page link used ./rockchip without the .md suffix. Docusaurus treats it as a plain relative URL; from /software-support-matrix (no trailing slash) it resolves to /rockchip → 404. Correct target is /software-support-matrix/rockchip. Changed to ./rockchip.md so Docusaurus resolves it to the real permalink.

2. Ordering within the section sidebar

   • Overview now has sidebar_position: 1, Rockchip sub-matrix sidebar_position: 2, so the overview appears first in the autogenerated sidebar.

zh/en updated in sync.