Snippet index: support for manual opt-outs

[teh-cmc]

Adds the ability to manually opt-out of the index for specific snippets/archetypes/components.

E.g.:

# These snippets will be excluded from the snippet index, unconditionally.
[snippets_ref.snippets.opt_out]
"archetypes/manual_indicator" = []

# These archetypes will ignore the associated snippets in the snippet index.
[snippets_ref.archetypes.opt_out]
"DataframeQuery" = [
  "reference/dataframe_save_blueprint"
]

# These components will ignore the associated snippets in the snippet index.
[snippets_ref.components.opt_out]
"ShowLabels" = [
  "tutorials/data_out",
]

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 Introduce the Snippet Index #8383

[github-actions]

Web viewer failed to build.

| Result | Commit | Link | Manifest |
| ------ | ------- | ----- |
| ❌ | | https://rerun.io/viewer/pr/8385 | +nightly +main |

^{Note: This comment is updated whenever you push a commit.}

[Wumpf]

lgtm