[SPARK-59033][INFRA][DOCS] Coverage badges at branch root + fix udfs page rendering - #59

…branch root

The README shields.io endpoints read .../badges/coverage-rust.json (root of the
badges branch), but coverage.yml wrote the JSON into a badges/ subdirectory, so the
published path was .../badges/badges/coverage-rust.json and the badges 404'd
('resource not found'). Write coverage-rust.json / coverage-python.json at the
branch root instead.

Co-authored-by: Isaac <no-reply@databricks.com>

The udfs docs page mangled type names (Vec<T>, Option<T>, Vec<Option<String>>,
udf::<name>): two inline code spans wrapped across a line, so with md_in_html the
bare <...> after the unclosed span were parsed as HTML tags and dropped. Collapse
those spans onto one line. Also drop the obsolete
'export SPARK_CONNECT_WASM_PACKER_PATH=...' line from the run example — the packer
is now embedded in the crate, so no path is needed.

Co-authored-by: Isaac <no-reply@databricks.com>

The four ```rust,ignore fences on the UDFs page never rendered as code:
pymdownx.superfences rejects a comma in the fence info string, so the
fence isn't recognized at all - the backticks show up literally and the
`#[spark_wasm_udf]` / `#[cfg(...)]` lines inside get parsed as H1
headings (see the reported screenshot). `rust,ignore` is rustdoc/mdBook
doctest syntax, not mkdocs; the site just wants a language, so use
```rust.

Verified by rendering the whole page through the site's exact extension
set: 7 code blocks, no stray backticks, no bogus headings.

Co-authored-by: Isaac <no-reply@databricks.com>

…e check

My previous fix flipped the UDFs page from ```rust,ignore to ```rust so
superfences would render it. That also opted those blocks INTO
check_doc_snippets.py (which compiles every plain ```rust block), and
they can't compile in that harness -- they use the `wasm-udf`
`#[spark_wasm_udf]` macro, a build.rs embedding step, and the generated
`udf::*` module -- so the Docs job's snippet-compile step failed.

Use the ```{.rust .no-run} brace fence instead: superfences still renders
it as a Rust code block (verified through the site's exact extension
set), but the checker's `^```rust$` regex doesn't match it, so it's
skipped -- the renderable equivalent of rustdoc's `ignore`. Document the
convention in check_doc_snippets.py so `rust,ignore` (which breaks
rendering) isn't reintroduced.

Co-authored-by: Isaac <no-reply@databricks.com>