docs/user: add docs for riscv/kendrytek210 BSP variant

This patch adds the documentation for building and running RTEMS on the Kendryte K210 RISC-V SoC.
The generic riscv introducion was re-arranged to list the multilib variants then the specific
hardware targets. In addition a couple of errors were fixed for the generic QEMU commands.

V2 corrected a typo, expanded K210 Console UART parameters, and addded a hyperlink to renode.io install
instructions.

V3 clarified the multilib variant description, clarified the multilib variant reference platform, and
corrected capitalization on SiFive.

V4 improves the instructions for running the K210 BSP on the Renode.io simulator.

V5 cleaned up the text to be no more than 80 characters per line.

V6 applied word wrap to paragraphs and replaced hard coded RTEMS major versions with macros.

Closes #4876

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