Plugin Chapter update

[NotTheDr01ds]

• Also updates DataFrames chapter to remove erroneous installation information. Points back to Plugins chapter for installation.
• Moves the "implementation" paragraph from the top down to the developer section
• Adds a callout on version matching near the top
• Trying out the automatic [[toc]] here (and in DataFrames). This populates a table-of-contents with links to each heading. We can easily (and IMHO should) add this to more chapters in the documentation.
• Adds a section listing the Core plugins that ship with Nushell and how to use them (addresses Document "default" plugins #1514).
• Changes code blocks to remove > prompt to match style guide.
• Reworks heading titles and levels for consistency and flow
• Other miscellaneous improvements

[fdncred]

There were some minor things that I was unsure about like "add a plugin" vs "register a plugin" but as a whole it's better than what we had. We can quibble about details some other time.