[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/

0 files changed, 0 insertions, 0 deletions