Search: add relevancy to API docs to prioritize definitions

[Eric-Arellano]

Meilisearch has a mechanism for relevancy, where you can have one search result win out over the other: https://www.meilisearch.com/docs/learn/core_concepts/relevancy.

This would be useful for API docs. It's common for us to have the same API like my_method show up both as a definition and also in references from other parts of the API docs. We should prioritize the definitions to show up before references.

This requires having structured metadata about where APIs are defined, which we can probably get from the upcoming API components like <Function>: #1008.