Add docsearch:description meta tag

[wackerow]

Description

• Adds a meta tag for docsearch:description to gain access to the current metadata description for each page via Algolia DocSearch
• About: https://docsearch.algolia.com/docs/required-configuration/#introduce-global-information-as-meta-tags

[wackerow]

This is in attempt to bring the new DocSearch index crawler config closer to what we currently have on the site, which utilizes the metadata description.
Currently:

    "lvl2": {
      "selector": "//meta[@name='description']/@content",
      "type": "xpath",
      "global": true
    },

...but the new crawler does not support that selector syntax.

[pettinarip]

Nice @wackerow!

I was wondering if we should also add the pathname as a top level selector. For example, when you search "Get ETH" (which is the name of the page in the nav dropdown), that name does not appear in the h1 or title of that page, ignoring it from the top search results you get.

Just an idea. A different workaround could be to do something like what we are doing in the Stablecoins page where we set the same name on the h1 as in the nav's dropdown name.