Doc/Context/Irmin: Intermediary between Tezos and Irmin

What

This MR is the starting point of the documentation regarding Irmin as used in Tezos.

Why

Irmin needs to be improved, some bugs need to be fixed, some modules need to be simplified but it is currently too hard to work with it as Irmin was mostly considered as black box.

How

This MR will be attached to other ones (mainly one where a proper test suite is implemented for Irmin and one where some module shadowing and naming are removed or replaced for readability -- without changing the code)

Manually testing the MR

This MR can't be tested as it is mere documentation.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR