Unify categories

[petrasovaa]

Currently we have:

Advanced (4)
Basic (4)
GRASS GIS (14)
Get started (1)
Google Colab (1)
Import (1)
Install (1)
Intermediate (6)
Jupyter (3)
Plotting (1)
Python (11)
R (2)
Time series (7)
Windows (1)
boxplot (1)
histogram (1)
line (1)
raster (7)
rgrass (1)
vector (1)

We need to decide:

1. lowercase or upper case? E.g. words that are not names could be lowercase (time series, plotting, ...)
2. basic, intermediate, advanced - how about beginner instead of basic?
3. Get started - is it separate category or should it be merged with basic/beginner?

We probably shouldn't overdo it with the number of different categories so that users can quickly see all the topics since the categories is the main way to navigate between the tutorials at this point.

[petrasovaa]

And also we should probably remove the GRASS GIS category as redundant

[veroandreo]

We need to decide:

1. lowercase or upper case? E.g. words that are not names could be lowercase (time series, plotting, ...)

lowercase

2. basic, intermediate, advanced - how about beginner instead of basic?
3. Get started - is it separate category or should it be merged with basic/beginner?

beginner for 2 and 3