docs: add pytest/libiio themed logo and favicon for Sphinx documentation

[tfcollins]

Summary

Adds a custom branded logo and favicon to the Sphinx/Furo documentation,
combining the pytest and libiio visual identities into a cohesive banana
mascot theme.

Changes

New Assets

• docs/_static/logo-pytest-libiio-banana.svg: Custom SVG logo featuring a
  banana mascot with purple goggles, green pytest checkmarks, circuit board
  traces, and signal waveforms — blending the pytest and libiio project
  aesthetics
• docs/_static/favicon.svg: 32×32 SVG favicon using the same
  banana+checkmark+goggle motif, with dark-mode support via CSS media queries

Styling

• docs/_static/custom.css: Constrains the sidebar logo dimensions to a
  maximum width of 120px and height of 100px to ensure consistent rendering
  across viewport sizes

Configuration

• docs/conf.py: Updates Furo html_theme_options to wire up
  light_logo, dark_logo, html_logo, html_favicon,
  html_static_path, and html_css_files so the new assets are loaded
  automatically by the theme

Impact

The documentation now has a distinct, project-specific visual identity that
reflects both the pytest testing framework and the libiio library. The
favicon improves browser tab recognition, and the SVG format ensures crisp
rendering at all resolutions including HiDPI displays.

Closes https://github.com/tfcollins/tbot/issues/19

Summary by Sourcery

Add custom branded pytest/libiio banana-themed logo and favicon to the Sphinx/Furo documentation and configure the theme to use them.

New Features:

• Introduce project-specific SVG logo and favicon assets for the Sphinx documentation theme.

Enhancements:

• Configure Furo theme options and static assets to use the new logo, favicon, and custom stylesheet for consistent branding across light and dark modes.

Documentation:

• Apply custom styling to constrain the sidebar logo size so it renders appropriately within the documentation layout.

[sourcery-ai]

Reviewer's Guide

This PR updates the Sphinx/Furo documentation configuration to use new custom pytest/libiio-themed SVG logo and favicon assets, and adds a small CSS override to constrain the sidebar logo size for consistent rendering.

File-Level Changes

Change	Details	Files
Configure Sphinx/Furo to load custom logo, favicon, and CSS from the _static directory.	Set html_static_path to include the _static directory so custom assets are served. Defined html_theme_options with light_logo and dark_logo pointing to the new SVG logo and enabled sidebar_hide_name. Configured html_logo and html_favicon to reference the new logo and favicon assets in _static. Registered custom.css in html_css_files so Furo loads the stylesheet.	docs/conf.py
Add CSS to constrain the sidebar logo size for better layout within the Furo sidebar.	Target the .sidebar-brand img selector used by Furo’s sidebar branding area. Limit logo to a maximum width of 120px and height of 100px while keeping aspect ratio via auto width/height.	docs/_static/custom.css
Add pytest/libiio-themed SVG logo and favicon assets for documentation branding.	Create a main banana-mascot SVG logo asset for use in the sidebar and header. Create a matching 32x32 SVG favicon asset designed to render crisply at small sizes and support dark-mode via CSS.	docs/_static/logo-pytest-libiio-banana.svg docs/_static/favicon.svg

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