Docs: Bring Javascript Style Guide in line with docs standards

What does this MR do?

The Javascript Style Guide did not match current docs standards. HTML was removed, and overall layout was simplified and made to match normal Docs style.

Since the HTML anchors were removed and section titles were changed, I did a search and found no references to them anywhere else in the docs.

Related to: https://gitlab.com/gitlab-com/gitlab-docs/issues/330

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards