Snippet index: support for enumerating custom features

[teh-cmc]

Adds the ability to manually enumerate custom things.

E.g.:

# These arbitrary feature names will be indexed exactly as written down.
# Add anything you can think about!
[snippets_ref.features]
"Dataframes" = [
  "reference/dataframe_query",
  "reference/dataframe_view_query",
]
"`AnyValue`" = [
  "tutorials/any_values",
  "tutorials/extra_values",
  "howto/any_values_send_columns",
]

Config will have to be filled over time!

Rendered

• Part of Examples: feature matrix table to know what's where #1123
• Part of Automatically generate list of used archetype in each example #5662
• DNM: requires Snippet index: support for manual opt-outs #8385

[github-actions]

Web viewer failed to build.

| Result | Commit | Link | Manifest |
| ------ | ------- | ----- |
| ❌ | | https://rerun.io/viewer/pr/8386 | +nightly +main |

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