extended "web frontend version"-based folder structure to HTML files

[u-an-i]

Hi @albar965 ,

when re-reading the README_documentation.txt of the web frontend I noticed a new reader can wonder why the HTML files are not grouped by web frontend version but the other files are.

And I thought having these grouped too is actually a good idea.

So I did. I adjusted all paths where necessary and tested all features successfully.

I think the web folder is now much less intimidating for new readers.

1 caveat is that the url for the current version now gets redirected to /html/2021-a/index.html and appears as such in the web browser address bar.
If you prefer to have the old /index.html only in the address bar, I think I can solve this in 1 of 2 ways:

• replace the url inside the address bar after redirection (yes, this is possible with JavaScript within same domain)
• use an iframe in /index.html which points to the current version rather than a redirection

[u-an-i]

@albar965 you can merge this if you like