Changes

Document Blocks and Components including file structure, templates, types and use of <Blocks />.
Adds references to new documentation page from main readme and relevant src directories.

Note: already references files, yet to be merged in #78 .

Associated issue

Part of #71

Review docs/blocks-and-components.md

[x] I have performed a self-review of my own code
[x] I have made sure that my PR is easy to review (not too big, includes comments)
~~I have added/updated tests to prove that my feature works (if not possible please explain why)~~
[x] I have made changes to the README and if the change affects the project setup (npm commands changed, new service added, environmental variable added)
~~I have added a decision log entry if the change affects the architecture or changes a significant technology~~
[x] I have notified a reviewer

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