Document SeedSigner architecture and flows

[etherbiswas]

Description

Problem or Issue being addressed

The current documentation lacked a detailed breakdown of structure for code, making it hard to make changes for new contributors/devs.

Solution

Modified README.md -- Now includes detailed breakdown using Mermaid flowcharts. Covers layer map, boot sequence, main user flow.

---

This pull request is categorized as a:

• Documentation

---

Checklist

I ran pytest locally

• N/A

---

I included screenshots of any new or modified screens

• N/A

---

I added or updated tests

• N/A

---

I tested this PR hands-on on the following platform(s):

• N/A

---

I have reviewed these notes:

• Keep your changes limited in scope.
• If you uncover other issues or improvements along the way, ideally submit those as a separate PR.
• The more complicated the PR, the harder it is to review, test, and
• We appreciate your efforts, but we're a small team of volunteers so PR review can be a very slow process.
• Please only "@" mention a contributor if their input is truly neede

• I understand

---

Description

Problem or Issue being addressed

Solution

Additional Information

Screenshots

---

This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Documentation
• Other

---

Checklist

I ran pytest locally

• All tests passed before submitting the PR
• I couldn't run the tests
• N/A

---

I included screenshots of any new or modified screens

Should be part of the PR description above.

• Yes
• No
• N/A

---

I added or updated tests

Any new or altered functionality should be covered in a unit test. Any new or updated sequences require FlowTests.

• Yes
• No, I’m a fool
• N/A

---

I tested this PR hands-on on the following platform(s):

• Raspberry Pi OS Manual Build
• SeedSigner OS on a Pi0/Pi0W board
• Emulator

---

I have reviewed these notes:

• Keep your changes limited in scope.
• If you uncover other issues or improvements along the way, ideally submit those as a separate PR.
• The more complicated the PR, the harder it is to review, test, and merge.
• We appreciate your efforts, but we're a small team of volunteers so PR review can be a very slow process.
• Please only "@" mention a contributor if their input is truly needed to enable further progress.

• I understand

---

Thank you! Please join our Devs' Telegram group to get more involved.