Reconsider menu type and placement on the docs site

[bugfolder]

This is re-opening an issue that was addressed by #215, and is also touched on by #149. A very brief summary (apologies in advance if I miss anything):

• On the docs site, particularly in the API, there are pages whose content is so wide that (at least to some people) it's better to use a fly-out side menu, rather than a proportional-width menu block. An example is https://docs.backdropcms.org/form_api. (Pushing the menu block to the bottom of the page, which had been happening with some pages, is not very useful when the page content is really long.)
• Pages that are primarily tables also benefit from having the full width available to the table, rather than squeezing the table into 2/3 of the width, which happens with a menu block. Those, too, got a fly-out menu. Example being https://docs.backdropcms.org/api/backdrop/functions.
• There was some feeling that having the menu block always on the left was preferable to having it jump back and forth from side to side, depending on which page it was on; so we'd moved it to the left on just the docs site.
• But there then was strong feeling that having it switch from left to right depending on which version of *.backdropcms.org one was on was also jarring, and that having it on the left impeded usability.

Further details and discussion may be found in the two issues linked above.

So this issue's goal is to reopen the question of "what to do with the menus on the docs.b.org site" (and, if necessary or appropriate, on the other b.org sites), and, specifically, to get designer input and expertise on the question, with the goal of achieving good usability on all pages.

Soliciting input from @dariusgarza, @yorkshire-pudding, @jenlampton, @irinaz, and of course, anyone else is welcome to chime in. (Please mention if you have design experience!)