[MLIR] Make generated markdown doc more consistent (#119926)

A few changes to doc generation:

- All summaries are in italics.
- In general each optional block starts and ends with a newline.
- All table elements are enclosed in `|`'s
- Overall reduce the number of >2newlines in a row

Rationale for this change is that our markdown to docs generator
requires a newline before all headers, otherwise it gets inlined into
the line before it, see `### sdy-op-priority-propagate` in the image
below.

<img width="883" alt="image"
src="https://github.com/user-attachments/assets/b795c424-cecb-48df-abbe-aee2030f4491"
/>

That said overall I feel this formatting is more consistent now, here's
a before and after:

- Dialect documentation diff: https://www.diffchecker.com/OVMHoXeL/
- Pass documentation diff: https://www.diffchecker.com/XEJRmW3k/
6 files changed
tree: 12c6f32461f6542e13ae9c95477d140ce946dcb0
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. libc/
  11. libclc/
  12. libcxx/
  13. libcxxabi/
  14. libunwind/
  15. lld/
  16. lldb/
  17. llvm/
  18. llvm-libgcc/
  19. mlir/
  20. offload/
  21. openmp/
  22. polly/
  23. pstl/
  24. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-tidy
  29. .git-blame-ignore-revs
  30. .gitattributes
  31. .gitignore
  32. .mailmap
  33. CODE_OF_CONDUCT.md
  34. CONTRIBUTING.md
  35. LICENSE.TXT
  36. pyproject.toml
  37. README.md
  38. SECURITY.md
README.md

The LLVM Compiler Infrastructure

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