Added Spring design doc

[olganaumenko]

Description

@EgorkaKulikov, please review the Markdown-formatted design document.

How to test

The PR introduces no code changes.
Manual scenario: the text itself has been previewed in the IntelliJ IDEA viewer.

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.