docs: make code highlighting look better

[hsheth2]

I picked these syntax themes because they looked good, but am not particularly attached to them - we can definitely change it around. Specifically, the vscode light theme's background color feels a bit too light. Docusaurus uses some custom themes that I think look pretty good - maybe we can use those?

Checklist

• [ ] The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• [ ] Links to related issues (if applicable)
• [ ] Tests for the changes have been added/updated (if applicable)
• [ ] Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• [ ] For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[github-actions[bot]]

Unit Test Results (build & test)

622 tests  +1   618 :heavy_check_mark: +1   15m 33s :stopwatch: -10s 158 suites +1       4 :zzz: ±0  158 files   +1       0 :x: ±0 

Results for commit 7ea8042d. ± Comparison against base commit e5a823e0.

:recycle: This comment has been updated with latest results.

[maggiehays]

the vscode light theme's background color feels a bit too light. Docusaurus uses some custom themes that I think look pretty good - maybe we can use those?

+1 - it's way too light & tough to read. Let's try Docusaurus's theme!

[hsheth2]

@maggiehays decided against changing the themes right now - main issue was (and still is) missing syntax highlighting for certain keywords