Add Rust docs

[powergee]

• Embeded HP(++).
  • https://github.com/kaist-cp/hp-plus is going to be archived after merging this PR.
• Removed -rs from SMR directory names.
• Grouped SMR folders in a separate subdirectory (smrs).
• Added CI badge and rustdoc.
• Docs Preview: https://powergee.github.io/pebr-benchmark/smr_benchmark/index.html
  • Note: The newly added badge for Rust Docs will not be rendered properly, because the link refers to the kaist-cp/smr-benchmark, which has not generated a doc yet. I have tested that the badge is rendered properly with a link to a forked one (powergee/pebr-benchmark).

[powergee]

The deployment failure is due to the environment protection rule for GitHub Pages, which can be configured Settings > Environments > github-pages > Deployment branches and tags. I think it must be done after merging.