Future documentation improvements

[bcantoni]

As a follow-on to some ideas discussed while doing edits in #462, here are some future ideas for some editing and other improvements we can consider:

• revisit Tableau version numbers which are sprinkled throughout but refer to versions no longer supported by Tableau; for example "Sandboxed Extensions are supported in Tableau 2019.4 and later" is technically true, but the oldest supported version today is 2020.4, so should we update those references? (and if so, should we do that each quarter when another older release is dropped from support)
• add pointers where to get help (DataDev Slack?)
• add consistent "what's next" and the end of each page, if appropriate
• fix problem with reaching tutorial pages from search (e.g. search for jQuery and click on "part 3")
• review target=blank usage for consistency and correct link construction (currently many of them don't work as intended)
• update screenshots with more recent version of Tableau
• review/remove emails like [email protected] and [email protected]
• review/update any remaining references to Extension Gallery
• consider running a markdown lint/formatter to keep style consistent