Switch images based on theme

[Blind4Basics]

Testing for #71 (waiting for the rendered preview version to see if that works... it doesn't x) )

[Blind4Basics]

'doesn't seem to work... :/

What did I mess?

[Blind4Basics]

...tried adding the ./img folder at root level: doesn't work either... (desperate fail, desperate attempt... XD )

(note: some files to remove from that branch, when it's done)

[kazk]

Apparently we can't use <img> tag for relative path. We might need to do:

<div class="block dark:hidden">

![alt](./light.png)

</div>
<div class="hidden dark:block">

![alt](./dark.png)

</div>

for now. I might add Markdown extension to add custom classes.

Can you also use lowercase extension .png?

[Blind4Basics]

looks like it's ok, now :)
All changed and I removed all the pending files/folders.