Create a select box to switch between releases/branches like the old docs

[mrpollo]

There used to be a very nifty feature in the old documentation, on the sidebar we had a select box populated with the latest releases, and it would let you jump between them.

• Fetch JSON dynamically from GitHub that had a list of version text and versions. This then applies to ALL versions of the docs, even ones that have already been published.
• When the menu is "closed" it displays the currently selected version - ie on v1.11 the text displayed in the box would be v1.11.
• Changing to new version changes to the same page in the new version

It looks like this on v1.10 branch (Try it here):

• For gitbook the version was fetched from master branch book.json, which looks like this. NOte the gitbookConfigURL URL is used if defined - this points to the "actual" version of the file that should be used, if it exists. On master it uses the other options.

      "versions": {
          "gitbookConfigURL": "https://raw.githubusercontent.com/PX4/px4_user_guide/master/book.json",
          "options": [
              {
                  "value": "https://docs.px4.io/master/en/",
                  "text": "master"
              },
              {
                  "value": "https://docs.px4.io/v1.11/en/",
                  "text": "v1.11 - stable"
              },
              {
                  "value": "https://docs.px4.io/v1.10/en/",
                  "text": "v1.10"
              },
              {
                  "value": "https://docs.px4.io/v1.9.0/en/",
                  "text": "v1.9"
              },
              {
                  "value": "https://docs.px4.io/v1.8.2/en/",
                  "text": "v1.8"
              }
          ]
      }