Restore buildSearchablePursDocs and launchSearchablePursDocs

[klntsky]

In the past, we used to download docs-search from NPM, but now that docs-search moved to spago repository, it is outdated. And we can't use it, because spago does not expose its interface directly, and --offline flag is not yet available in a released spago version.

As a result, we remove searchable docs functionality in #1521 - it has to be restored when there is a new spago release