Add RISC-V 64-bit (riscv64) platform support

[gounthar]

The state API does not include LINUX_RISCV64 in its Platform enum or OpenAPI spec.

Changes needed

1. Domain.kt: add LINUX_RISCV64("linuxriscv64") to the Platform enum
2. documentation.yaml: add LINUX_RISCV64 to both platform enum lists in the OpenAPI spec

Related

• feat: add RISC-V 64-bit platform support sdkman-cli#1496, feat: add RISC-V 64-bit platform support sdkman-cli-native#375, feat: add RISC-V 64-bit platform support sdkman-disco-integration#46, feat: add RISC-V 64-bit (riscv64) platform support vendor-release#17, feat: add RISC-V 64-bit (riscv64) platform support sdkman-broker-2#10