Integrated lunr search and fixed MathJax

[heethesh]

MathJax

Note that this PR overrides _includes/scripts.html in the remote theme. So the contents of the original file in the remote theme is copied and needs to be kept up-to-date from here.

• Added a flag site.math for math enable in _config.yml.
• Updated docs to use $..$ for inline math and $$...$$ for block math. This worked where \\(...\\) failed in some cases. The latter syntax is also still supported though.
• Fixed few articles that had incorrect Latex syntax and did not escape some characters.

Lunr Search

• Added support for indexing pages in assets/js/lunr-store.js. The default lunr search only supports indexing collections.
• Improved search indexing by removing markdown special characters.
• Removed the old Google search plugin and related files. If needed, Jekyll supports Google search directly and should be enabled by setting the search provider within _config.yml.

Tested locally on Jekyll server. Everything else looks fine.