docs: Add architecture information to README.md

[mbonica90]

Description:
Added a description of Transaction Tool's frontend and backend architecture to the README.

Notes for reviewer:
README-only change.

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.) (N/A, docs only)

[swirlds-automation]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.98%. Comparing base (c48a7da) to head (fe2231c).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3339   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files         208      208           
  Lines        6506     6506           
  Branches     1197     1172   -25     
=======================================
  Hits         6505     6505           
  Misses          1        1           

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