nvx-rs: keep the README's module table to one short line per module, and describe each module after it #964

Summary

The module table in nvx-rs/README.md (| Module | Contents |, under Layout) is no longer an overview. Each of its 45 rows is one Markdown line that holds the module's whole description: 42 rows are longer than 200 characters, 14 are longer than 1,000, and the table holds about 49,700 characters, 38% of the README. The longest rows are nvx::host::template (7,130 characters), nvx::guest::root (3,969), nvx::host::sandbox (3,638), nvx::guest::clone (3,587) and nvx::activation (2,235).

Rendered, a reader scrolls through screens of one cell to reach the next module. In review, any change to a description replaces its whole line. #951 adds about 4,500 characters inside the nvx::host::template row, so its diff shows a line of 12,996 characters replaced by one of 17,503, and the change itself is hard to find. Open pull requests keep growing these rows: 16 of them edit the rows of nvx::host::template, nvx::guest::clone, nvx::host::sandbox or nvx::guest::mappings.

Proposal

• Make each row of the table a short summary of what the module is for: one sentence of at most 200 characters, with a link to the module's section. The rows of nvx::Hypervisor, nvx::host::cli and nvx::guest already have this form.
• Move each module's details into a section of its own after the table, for example ### nvx::host::template, as prose wrapped like the rest of the README, with a paragraph or a list item per type or operation, so that a change touches only the lines it changes. The move keeps the current wording; shortening the descriptions can follow separately.
• Say above the table that a module's details go in its section, and that its row changes only when the module's purpose does. Add the same rule to .github/copilot-instructions.md.

A check that fails when a row of the module table grows past the limit would keep it short; it is optional. The README's other tables already keep their cells short and are out of scope.

Acceptance criteria

• No row of the module table is longer than 200 characters, and each links to its module's section.
• Every description that the table holds today is in a section after it, with its wording unchanged by the move.
• Links into the README, such as #distro-bindings and #checked-handoff, still resolve, and the Markdown link check passes.

Sequencing

The open pull requests that edit rows of the table (#888, #925, #926, #939, #940, #941, #949, #950, #951, #955, #956, #957, #958, #959, #960 and #961) conflict with this change in nvx-rs/README.md. Whichever lands second moves its README text into the module's new section.