docs: add external kernel module build steps

[tzz-radxa]

Summary

• Document the out-of-tree kernel module workflow for bsp.
• Add both cross-compilation and board-native build commands.
• Add the kernel/header version-matching and local-source preservation notes.

Source

This change comes from the confirmed support topic omt_19d5400daf8e5b93 on 2026-09-21, where a ROCK 5B+ user asked how to compile a custom driver in the Docker/bsp environment.

The existing kernel page explained how to build and install the kernel and headers, but did not show how to turn a driver source tree into a .ko module or how to build it against /lib/modules/$(uname -r)/build. That omission made the supported workflow difficult to discover.

Checks

• git diff --check
• pre-commit run --files docs/common/dev/_kernel.mdx i18n/en/docusaurus-plugin-content-docs/current/common/dev/_kernel.mdx
• python3 scripts/github_pr_guard.py check --repo-path <worktree> --fetch