doc: Fix Markdown formatting in expression language specification

[wForget]

Summary

Fix Markdown rendering issues in core-spec/expression_language.md:

• Escape the || concatenation operator inside Markdown tables to prevent it from being interpreted as a column separator.
• Add spacing before section dividers so they render correctly.
• Make no semantic changes to the expression language specification.

Related Issues

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Pull request overview

Improves Markdown rendering in the expression language specification by adjusting table content and section spacing so the document renders correctly on GitHub without changing the intended semantics.

Changes:

• Added blank lines before horizontal rules to ensure section dividers render properly.
• Updated occurrences of the || concatenation operator within Markdown tables to avoid being interpreted as a column separator.

Comments suppressed due to low confidence (1)

core-spec/expression_language.md:668

• This table cell will render the backslashes literally because it is inside an inline code span (showing a \\|\\| b instead of a || b). Use a <code> tag with | entities to keep the displayed syntax correct without breaking the Markdown table.

| String concat | `CONCAT(a, b)` | `CONCAT(a, b)` | `CONCAT(a, b)` | `CONCAT(a, b)` | `a \|\| b` |

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[wForget]

Reference: GFM Spec, §4.10 Tables, Example 200, which explicitly states that pipes can be escaped inside other inline spans and shows `\|` rendering as <code>|</code>.