Add description key and description processing to sources tab

[pabloarosado]

We used to show plenty of information about variables (including information about the processing of the variable) via the description field, which is surfaced in the sources tab. But now, with the new metadata, we are only surfacing descriptionShort, and therefore we are currently missing crucial information in charts.

This PR adds a section in the variable description table called "Key information" (showing descriptionKey) and another called "Processing notes" (showing descriptionProcessing).

NOTE: For some reason, descriptionProcessing is always empty in the database variables table, but I understand that this was unintended, and can be easily fixed. (Mojmir has fixed this)

[pabloarosado]

This is an example of how the sources tab will look like:

One thing to note is that bullet points within the string field descriptionProcessing (that in the YAML file were created using - Blah) look different to the bullet points of descriptionKey. But I guess this is a minor style concern.

[lucasrodes]

LGTM, thanks for the rapid implementation

[danyx23]

Very nice work - thanks Pablo! I added a new markdown renderer to fix some issues with broken list items.

Good to merge!