Add riscv64 CI build and release assets

Problem

mdBook has no CI coverage for riscv64 and no pre-built riscv64 binaries in releases. The CI already cross-builds for aarch64-unknown-linux-musl; riscv64 is a natural extension. Users on riscv64 hardware (SBCs, cloud instances, RISC-V workstations) currently build from source without CI validation.

Proposed Solution

Two additions:

1. CI (main.yml): add a riscv64gc-unknown-linux-gnu cross-build job (similar to the existing aarch64-cross-builds job), or use native RISE riscv64 runners (ubuntu-24.04-riscv) for build + test.

2. Release (deploy.yml): add riscv64gc-unknown-linux-musl (or gnu) to the matrix so releases include a riscv64 binary.

Runner access for the rust-lang org: install the RISE GitHub App. These are native riscv64 machines - no emulation needed.

I can open a PR if this gets the E-Help-wanted or Feature accepted label. Per CONTRIBUTING.md, I understand only labeled issues receive reviews.

Notes

I've built mdBook successfully on riscv64 hardware (SpacemiT K1, Banana Pi F3). cargo build --release and cargo test pass without code changes; Rust's stdlib supports riscv64gc out of the box.

Start by comparing the existing aarch64-cross-builds job in main.yml and the target matrix in deploy.yml. Check the available riscv64 target and runner options, then verify cargo build --release and cargo test for riscv64. Done means CI validates the target and releases contain a riscv64 binary.