odb: add LEF/DEF grammar railroad diagram docs

[Bbn08]

Add documentation pages for the LEF and DEF grammars with EBNF summaries and links to the interactive railroad diagram generator.

Resolves #2488

Description

Add documentation pages for the LEF and DEF grammar files with EBNF summaries
and instructions for viewing interactive railroad diagrams.

Each page includes:

• A brief introduction to railroad diagrams
• Top-level grammar structure for quick reference
• Links to the Railroad Diagram Generator
  for interactive viewing
• Links to the full EBNF grammar provided in Grammar railroad diagram #2488

Files Changed

• src/odb/doc/LEF_Grammar.md (new) LEF grammar documentation
• src/odb/doc/DEF_Grammar.md (new) DEF grammar documentation
• docs/toc.yml Added both pages under Database > ODB section

Design Note

Note

Rather than embedding the full EBNF grammars directly into the documentation pages (which would be very large), I opted for a lighter approach:

• Each grammar page (LEF_Grammar.md, DEF_Grammar.md) includes a top-level summary of the grammar rules for quick reference.
• The full EBNF (originally provided by @mingodad in Grammar railroad diagram #2488) is linked rather than duplicated, keeping the docs concise and the PR small.
• Users are directed to the Railroad Diagram Generator to paste the EBNF and view interactive diagrams.

If you'd prefer the full EBNF grammars included directly in the Markdown files (or as separate .ebnf files committed to the repo), I'm happy to update the approach.

[gemini-code-assist]

Code Review

This pull request is a valuable addition, providing clear documentation for the LEF and DEF grammars. Using an external tool for interactive railroad diagrams is a smart approach to keep the documentation concise and user-friendly. The new markdown files are well-written and the changes to toc.yml are correct. I have one minor suggestion regarding a typo in DEF_Grammar.md that originates from the source code.