Fix plantuml

[stueynz]

The Addinig Diagrams and Reading IGs pages weren't in the menus - now they are.
Added note about maybe needing to use {::nomarkdown} and {:/} to wrap the {% include } tag that includes ..svg files generated from plantUML

[costateixeira]

1. do we need to change the scripts? They were working - if there is a need for the scripts, they are here: https://github.com/HL7/ig-publisher-scripts
2. I think you tested the nomarkdown and it may not be needed, but depends on the jekyll version - do we want to explore further? Eg. instead of opening the nomarkdown tag and closing it at the end, why not just add it a the end like we do with tables?
3. The pages for diagrams were in a second level - i.e. inside another page - yes, a bit less visible. We can change it , but wanted to let know that this was by initial design - not all pages have to be at top level. But I think it's ok to add more visibility.
4. In general - perhaps best to make different PRs for the features (i.e. scripts+ page layout + styles) so that we can accept them in parts, instead of rejecting it all because one small thing that may not be easily agreeable.