Style rule: prefer index directives over index roles in headings

[mmcky]

Proposed rule

Prefer {index} directives over {index} roles in section headings.

Problem

Using the index role inline in a heading makes the title hard to read in the source:

# {index}`Polars <single: Polars>`

Preferred style

Use a plain heading followed by one or more index directives:

# Polars

```{index} single: Polars
```

Rationale

• The heading text stays clean and readable in the .md source
• The index entries are still generated identically in the built output
• Easier to maintain when multiple index entries are needed (just add more directives)

This came up during review of QuantEcon/lecture-python-programming#408.