Add missing GUI documentation HTML files to docs/

[Copilot]

The README links to GUI documentation at https://skblaz.github.io/py3plex/gui.html, but the HTML files were missing from the docs/ directory published to GitHub Pages.

Changes

• Added docs/gui.html, docs/gui_architecture.html, docs/gui_testing.html - generated from existing RST sources in docfiles/
• Added docs/sir_epidemic_simulator.html - also missing from docs/
• Updated docs/index.html to include "GUI (Web Interface)" navigation section

Context

The GUI RST source files existed in docfiles/ and were properly configured in Sphinx, but the built HTML artifacts were never committed to docs/. This PR copies the Sphinx build output to make the documentation accessible at the published URL.

Original prompt

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This section details on the original issue you should resolve

<issue_title>gui docs</issue_title> <issue_description>gui docs not accessible from readme</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes SkBlaz/py3plex#445

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