Use mkdocs multi-repos plugin

[noandrea]

In order to keep elesto documentation as close to source code as possible, while still having a single documentation entrypoint:

• the website portal will be elesto-docs
• the documentation can be in the /docs folder of elesto (or other projects)
• the documentation of each module can be in the x/module/ folder of elesto

Use the mkdocs multi-repos plugin to import the documentation and publish it into a single website.

[github-actions[bot]]

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