feat: Version switcher

[michael-wisely-gravwell]

This PR partially addresses #838

This PR proposes adding a version switcher to the docs navbar

Couple notes:

1. We're not currently serving the versions.json file at docs.gravwell.io. As such, when you build with make clean livehtml, you'll see something like this instead of a nice version switcher. You can fake it with the following patch:

   diff --git a/conf.py b/conf.py
   index 40ac78d..321d225 100644
   --- a/conf.py
   +++ b/conf.py
   @@ -96,7 +96,7 @@ html_theme_options = {
        # Version switcher
        #
        "switcher": {
   -        "json_url": "https://docs.gravwell.io/_static/versions.json",
   +        "json_url": "http://localhost:8000/_static/versions.json",
            # The `version` field of each entry in verions.json must match a vN.N.N release  name
            "version_match": release,
        },

2. Maybe it's just me, but the gravwell logo looks vertically misaligned? Like the center-of-mass is too high or something

   

[ashnwade]

regarding 2, I think that you would like the alignment better if the bottom of "ravwell" was aligned with the bottom of the version switcher box ("g" has the tail going below the line - the line is not visible, but your mind is putting it there), but then the menu header at the top would probably need to be taller so that the "g" isn't outside of the header.

[ashnwade]

LGTM