Make docs pages under /documentation/ searchable

[jasalt]

Currently the site search indexes only https://phel-lang.org/documentation/api/ entries but it would be very useful to also include the other documentation pages in https://phel-lang.org/documentation/.

Example documentation site with search returning both docs and function reference page entries grouped in quite clear way https://timber.github.io/docs/v2/. It uses Algolia, info.

Looking at the elasticlunr the current site search uses (background e.g. at #25), which I'm not familiar with, I'm left interested if the used search indexing and search result grouping and sorting could be accomplished in similar fashion.

Configuration-wise, maybe the (current) API / function reference entries should nevertheless be first in search results, in contrast to Timber docs where such entries have lower priority.