Initial docs for SQL syntax

[Dandandan]

Which issue does this PR close?

Closes #241

Rationale for this change

It would help users that use the sql syntax if we have a documented reference

What changes are included in this PR?

Initial setup for documenting the sql syntax.

Are there any user-facing changes?

No, if the book is published, this allows for more discoverability into the syntax.

Screenshot:

[codecov-commenter]

Codecov Report

Merging #242 (fb0178b) into master (c945b03) will not change coverage.
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[alamb]

Thanks @Dandandan -- I also think we should strive to (re)use as much other standard SQL as possible (aka not aim to write a complete SQL reference book). Introductory material (such as this PR) as well as datafusion specific things make a lot of sense to me.