Cranelift modernize docs

[AnnieSld]

I'm reopening this as a new, smaller PR.
This PR contains only the changes I verified against the current codebase:

1. cranelift/docs/testing.md: removed test commands that no longer exist in cranelift/filetests/src/ (test legalizer, test regalloc, test simple-gvn, test licm, test dce, test shrink, test simple_preopt). Also, updated outdated file paths referencing the old cranelift-codegen/meta-python/ structure, and replaced broken Sphinx syntax with standard Markdown.
2. cranelift/docs/add-clif-instruction.md: new guide documenting how to add a new opcode to CLIF, verified against cranelift/codegen/meta/src/shared/instructions.rs.
3. cranelift/docs/isle-integration.md: updated a paragraph about move mitosis that stated regalloc2 was a future migration; regalloc2 is already integrated in the current codebase.

Addresses #4135