Add docsearch to pkgdown site

[nuest]

https://pkgdown.r-lib.org/articles/search.html?q=search#indexing

• submit pkgdown site to docsearch
• get JavaScript code/tokens from docsearch team
• add configuration

docsearch rules: https://community.algolia.com/docsearch/who-can-apply.html

[nuest]

Congratulations, your search is now ready!
I've successfully configured the underlying crawler and it will now run every 24h.

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

• Use these credentials as explained here:

apiKey: 'a23b40c20595eb644e96b1f03f833bfe',
indexName: 'sos4r',

• Link to create an account for [email protected] is https://www.algolia.com/users/invitation/accept?invitation_token=YPPqY4-ZP7r_brDBf-en
• Optionally customize the look and feel by following the DocSearch documentation
  (See https://community.algolia.com/docsearch/styling.html)
• You can also check your configuration in our GitHub repository
  (See https://github.com/algolia/docsearch-configs/blob/master/configs/sos4r.json).