Improve `content/documentation/` indexing and search configuration

[jasalt]

PR #128 brought the initial support of indexing markdown documentation contents and adding them to website's search index. This should be optimized further based on notes in #128 (comment) and actual usage experiences regarding:

• Quality of markdown indexing done in composer build (might not include everything wanted as of now)
• Tuning of search.js parameters for optimal search behaviour
• Style/layout of the search result item type ("Documentation:" / "API:") could take less space as small text on it's own line above result
• Possible code refactoring

Feedback on the site search feature is welcome.

[jasalt]

As examples on current problems, searching of configuration option by name SrcDirs that is documented in https://phel-lang.org/documentation/configuration/ does not come up in search:

Searching text from source code snippets does not seem to work either: