Revise documentation before release

[gizmecano]

In order to make clearer the separation between the references required for the different variants, the location of the general documentation should be revised.

• [x] docs: contains README.md documentation files in English and French, as well as a list of dissimilar files between the variants and the compatibility table with OpenCart core versions
• [x] .github: contains only templates for issues, pull requests, etc.

In addition, links concerning these variants may be added, permanently (or only temporarily):

• [x] repository of the free variant
• [x] ~download page of the full version~ (cf. comment below)

[gizmecano]

Pending the final activation of an appropriate account, the full version cannot currently be released on a download page of the full version in the OpenCart Marketplace.