reference: cleanup syntax-and-data page

[l-zeuch]

Clean up the Syntax and Data page.

This PR will probably amass a rather large amount of commits, due to its
sheer size. I'll try to keep each individual commit's diff reasonably
small, starting with some cleaning of Gitbook's generated markdown.

To outline some of the changes I'll be making:

• Some paragraphs are repeated verbatim -- remove / rephrase duplicates.
• Some sections are far too verbose for an "overview" page -- shorten
  them, and/or move them to learn instead.
• Certain field descriptions could be more concise; opt for supplemental
  paragraphs below the table instead.
• Rewrite some data types to be more consistent with the already
  established terms in the learn section. There isn't a lot of reason
  to list the bot's internally used data types.

If you feel like submitting additional patches, please do so (raise them
against this branch). I'd greatly appreciate any work I do not have to
do myself.

Terms

• I have read and understood this project's Contributing Guidelines

[l-zeuch]

This has been sitting around far too long; I'll close this for the time being and maybe start over whenever I find the time.