mkdocs: Return to default plugins and site_url

This re-enables the search bar and makes the header logo link to docs homepage regardless of deployment location.

Test plan: Offline (local testing):

• follow the local testing instructions in doc/publishing-documentation.md in the 'Testing changes locally' section to set up a local instance
• browse to check that you have the Search bar available on top

Online:

• check that Search bar is available at https://bigblockiftrue.gitlab.io/bitcoin-cash-node/

For both cases (on-/off-line) check that the header logo links to the respective docs site (inspect the link)