Add descriptions to Graph Schema

[Jiros]

Add descriptions for each entity loaded in the CovidGraph Schema by the data loading/processing repositories below.

The template and naming conventions can be found on the Main Wiki

Data Loading

• Cord-19 (@motey)
• Cellmap Base (@mpreusse)
• Cellmap Annotation (@mpreusse)
• Cellmap GTEx (@mpreusse)
• John Hopkins/UN (@motey, @mpreusse)
• Covid Patents (@motey)
• Biobase
• Helomics

Processing

• Fragmentise Text (@mpreusse)
• Text Gene Match (@mpreusse)