Tracking Issue for changing rustc target docs

[Noratrieb]

This is the tracking issue for changing rustc target docs to be generated based on the existing target triple patterns, ensuring that there's a doc page for every single target.

Discussion in https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/generating.20target.20documentation with a draft implementation in https://github.com/Nilstrieb/target-tier-docs-experiment.

Steps

• Figure out the best format to store the target docs
  • Original idea: TOML
  • Current choice: Markdown with fixed sections and YAML frontmatter
• Move some of the information into the complier
  • Add empty target metadata to every target
    • Add a description field to target definitions #121905
    • Add the new description field to Target::to_json, and add descriptions for some MSVC targets #122157
    • Add metadata to targets #122305
  • Add unstable way to print descriptions
    • As it's contained in the target spec, we can just print the target spec JSON.
• Implement infrastructure for new docs (without using it yet) Introduce infrastructure for generating target docs #121051
• Migrate existing docs to the new format (over several PRs)
  • Fill out target-spec metadata for all targets #127265
• Use the new format for the shipped docs
• Delete the old docs

Future possibilities

• Keep tier, host_tools and std in sync with reality