Make the doc version selection drop-down box more obvious

[lilin90]

Some users didn't use the TiDB docs for the right version and this could lead to (minor) issues. Perhaps that's mostly because people were using Google search, which is somewhat improved by archiving older versions.

Maybe the version drop-down on the top left can be made to be a bit more visible by making it bold or bigger and/or using a different color?

[likidu]

Maybe we can consider to make the version selector fixed on the top of the ToC area then user can easily switch it.