fix: exclude non-let definitions from docs Functions index

[prql-bot]

Problem

prqlc experimental doc builds a "Functions" index that links to per-function detail sections. The index listed every VarDef, but the detail sections were only emitted for VarDefKind::Let. A top-level pipeline becomes a main var-def (and into produces another non-let kind), so those appeared in the index linking to an anchor that no detail section ever generated — a dangling link.

Reproduction (before this change):

$ printf 'let f = a -> a\nfrom employees | select {name}\n' | prqlc experimental doc
## Functions
* [f](#f)
* [main](#main)   ← links to #main, but no `### main` section exists

### f
...

The HTML output had the same issue with #fn-main.

Fix

The index loop and the detail loop each decided independently what counts as a documented function, and they had drifted. This consolidates that decision into a single is_documented_function helper used by both loops in both the HTML and Markdown generators, so the index can never list an entry the detail loop skips. The redundant stmts.clone().into_iter() passes for these loops are replaced with stmts.iter().

Test

Added generate_docs_excludes_main_pipeline, which runs experimental doc on input containing a top-level pipeline and asserts the main entry no longer appears in the Functions index. Without the fix the index regains the dangling * [main](#main) line and the test fails.

Found during the nightly code-quality survey.