Add shortcodes for image placement

[gmantele]

This PullRequest adds two shortcodes in order to help placing images in a page:

• floating.html: it is just applying the CSS style float: left/right on a given content. This content can be an image but also anything else. It may take one (un-named) parameter: left (default) or right

  Usage example:

  {{< floating right >}}                                                           
  ![](ivoa_logoc2.jpg)                                                             
  {{</ floating >}}
  

• side-image: this shortcode is entirely dedicated to images.

  It takes at least one parameter, the URL of the side image: image.

  It also accepts another parameter to give the URL of a thumbnail of the image: thumbnail. With this additional parameter, the image becomes clickable ; the click leads to the full size image.

  A third parameter lets specify whether the side image should be on the left or on the right: position. If unspecified, it will be on the left.

  The body content of this shortcode, is the content (text or not) to be placed on right or the left of the image.

  Usage example:

  {{< side-image image="my-image.png" thumbnail="my-image_thumb.png" >}}
  **Super** blabla
  {{</ side-image >}}
  

  This shortcode could be improved by adding a target URL. Then, the click would not lead to the full size image but to the given URL. The full size image could then be displayed on mouse hover or with a small magnifier button appearing over the image. Similarly, it could be possible to do the same with a figure (then, with a numbered caption) instead of a simple image. All these improvements (and maybe others) can be made later when needed.

Fixes #84

[molinaro-m]

@JeremyMcCormick @gmantele is this safe to merge? I'd like to try it for member organizations page and I'm wondering how to proceed

[gmantele]

It works fine with newsletters on my local preview but also on our test website:

• example of the floating shortcode: https://webtest.ivoa.info/v/iss40-add-newsletters/newsletter/025_march_2022/
• example of the side-image shortcode: https://webtest.ivoa.info/v/iss40-add-newsletters/newsletter/004_may_2010/

[molinaro-m]

I'd say we can approve this (for me it works).
Feel free to merge it, so we can build other pages on top of this.

[JeremyMcCormick]

Thanks for these!

Going to merge