[TechDocs] Implement a DefaultTechDocsCollator

What / why

In #4438, we introduced the concept of a Search collator, which is a class that a plugin can export whose purpose is to return all indexable documents the plugin is responsible for. We should implement a DefaultTechDocsCollator which returns documents representing individual TechDocs pages across every known TechDocs site.

Possible Implementation

• Iterate through the software catalog, looking for TechDocs annotated entities.
• The basis for the aggregated index should be the lunr index that is generated for us by mkdocs (e.g. the docs key here)
• The location field will need to configurable (recommend following the pattern in DefaultCatalogCollator
• Capture other potential metadata beyond the given title, text fields (like entity name/kind/namespace, etc.)

[Xantier]

I could grab this and start chipping away to bring the search to docs as well. I’ll start crafting types and shapes, open up a PR on those and we can then iterate from there with the implementation and tests to surround rest of the logic.

[dtuite]

@iamEAP Just FYI, Roadie (primarily Xantier) are planning on driving through the whole TechDocs/ElasticSearch implementation to completion. This is step 1 but we are hoping to get them all done.