Add dark mode support to documentation website

[GaneshPatil7517]

This PR adds optional dark mode support to the Apache Shiro documentation site.

Changes include:

• CSS variables for theme colors
• Dark theme overrides for Bootstrap components and Asciidoctor content
• Theme toggle button in the header navigation
• Persisted user preference using localStorage

The implementation is lightweight, does not introduce new dependencies,
and follows existing site structure.

Tested locally using Maven site generation.

Closes #266

[GaneshPatil7517]

Apache.Shiro.mp4

[Copilot]

Pull request overview

This PR adds optional dark mode support to the Apache Shiro documentation website and introduces a new troubleshooting FAQ page. The dark mode implementation is lightweight and follows modern web practices with CSS variables, localStorage persistence, and smooth transitions.

Changes:

• Added theme toggle functionality with CSS variables for theming
• Created comprehensive troubleshooting & FAQ documentation page
• Updated navigation menus to include troubleshooting link

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/site/assets/js/theme.js	Implements theme toggle logic with localStorage persistence and icon updates
src/site/assets/css/base.css	Defines CSS variables for light/dark themes and comprehensive dark mode overrides for Bootstrap and Asciidoctor components
src/site/templates/menu.ftl	Adds theme toggle button to navigation and troubleshooting link to Community dropdown
src/site/templates/header.ftl	Loads theme.js early in head to prevent theme flash on page load
src/site/content/troubleshooting.adoc	New comprehensive FAQ page covering common Shiro issues with practical solutions
src/site/content/support.adoc	Adds link to new troubleshooting page in support resources list

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[lprimak]

Hi, Ganesh!
Thank you once again for your contribution.

I deployed the staging website here: https://shiro.staged.apache.org

However, the dark mode toggle doesn't seem to work.
Never mind, I had to clear the cache and it works.

However, in dark mode, much text (announcements / blogs) for example is unreadable.

Can you please take a look?
Thank you!