(re) enable Algolia docs search for Akka HTTP

[ktoso]

Via Maxime from Algolia:

You're now a few steps away from having it working on your website:

• Copy the following CSS/JS snippets and add them to your page

<!-- at the end of the HEAD -->
<link rel="stylesheet" href="https://cdn.jsdelivr.net/docsearch.js/2/docsearch.min.css" />
<!-- at the end of the BODY -->
<script type="text/javascript" src="https://cdn.jsdelivr.net/docsearch.js/2/docsearch.min.js"></script>
<script type="text/javascript"> docsearch({
apiKey: '0ccbb8bf5148554a406fbf07df0a93b9',
indexName: 'akka-http',
inputSelector: '### REPLACE ME ####',
algoliaOptions: { 'facetFilters': ["version:$VERSION", "language:$LANGUAGE"] },
debug: false // Set debug to true if you want to inspect the dropdown
});
</script>

• Add a search input if you don't have any yet, and update the inputSelector value in the code snippet to a CSS selector that targets your input field.

• Replace $VERSION with the version you want to search on.
  The list of possible version is automatically fetched from your website.
  So as of today you have: 10.0.0, 10.0.1, 3.0.0-RC1, current

• Replace $LANGUAGE with the language you want to search on.
  The list of possible language is harcoded in the config.
  So as of today you have: java, scala

For example if you want the search on the version "10.0.0" and the language "java" just specify: 'facetFilters': ["version:10.0.0", "language:java"]

• Optionally customize the look and feel by following the DocSearch documentation (https://community.algolia.com/docsearch/documentation/)
• You can also check your configuration in the github repo (https://github.com/algolia/docsearch-configs/blob/master/configs/akka-http.json). Feel free to open pull requests!

[ktoso]

This is how we have it in Akka: https://github.com/akka/akka/blob/56a3718d0008ac446cb199c4add0cb42becbcf45/akka-docs/_sphinx/themes/akka/layout.html#L215