Ignore mdbook `theme/`

[rootulp]

Context

celestiaorg/celestia-node#3205 (comment)

highlight.js is the JavaScript that is used to highlight code snippets, you should not need to modify this.

Ref: https://rust-lang.github.io/mdBook/format/theme/index.html

Proposal

Investigate if the highlight.js in this repo differs from the default. If not, consider gitignoring it.

Ref: https://github.com/celestiaorg/celestia-app/blob/main/specs/theme/highlight.js