feat(docs): Add cardano-chain-follower architectural docs

[FelipeRosa]

Description

Architectural design docs cardano-chain-follower.

Related Issue(s)

Closes #67.

Description of Changes

Added the initial version of the architectural design docs for the cardano-chain-follower crate.

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[minikin]

@FelipeRosa Can you address comments, please?

[FelipeRosa]

@Mr-Leshiy I couldn't the docs for the miniprotocols (the link I had doesn't work anymore). Couldn't find anything specific to Mithril snapshots as well. So I linked to Cardano's network docs and Mithril's GitHub.

[Mr-Leshiy]

LGTM