Docs: Fix search in the docs on the web site
Context
Fixes #1563 (closed).
Search on https://tezos.gitlab.io/ is broken due to compatibility issues between latest version of sphinx and old version of sphinx-rtd-theme. Updating sphinx-rtd-theme to a later version (as suggested at https://github.com/sphinx-doc/sphinx/issues/8623) fixes the issue.
Also updated the way MichelsonLexer is passed into Sphinx - it takes class rather than instance since version 2.1 (https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.add_lexer). Without this fix docs were failing to compile with latest Sphinx (4.1).
Manually testing the MR
NOTE: if you have existing checkout with old version of Sphinx installed then you may need to run $ poetry install inside docs folder to reproduce the error.
Compiled docs locally ($ make inside docs folder) and checked in the web browser. Before the change - search is broken (with Uncaught ReferenceError: Stemmer is not defined error in JavaScript consoler) as shown in #1563 (closed). After the change - search is working. Other pages also look fine.
