Update documentation description

[firezym]

I try to update extension doc description by:

1. add a gif demo of how to use the extension by clicking button and drop down list

2. udpate the descriptions of ways to use the commands

3. optimize the package.json and schema\plugin.json 's descriptions so that users could find the extension by common keywords more easily in the discover list or setting's list.

[parmentelat]

Hey @firezym

thanks for this contribution
looking at you gif it occurred to me that there is room for improvement, in that the actions could work on all the selected cells, which would allow to change for example 2 consecutive cells at the same time
it is actually a trivial change to make, I'll open a separate issue about that, maybe once this is taken care of we could record the gif again to illustrate this future possibility

in any case for now I am merging, and will issue a new release soon, hopefully with the improvement above as well if I find the time

much apprecialted, thanks !

[firezym]

@parmentelat Sure, I will record again to update after it is done :) Thanks a lot!

[parmentelat]

am releasing 0.2.3 as we speak; if all goes well you should be able to use this to record again;
thanks !