Docs: Add documentation for Sphinx tags (#4603)

[Gunashekar31742]

Hi there! This pull request adds documentation for the Sphinx .. only:: directive (content filtering with tags), as requested in issue #4603.
I've added a new section to the features.rst file with a brief explanation and a code example to make it clear for users.

Closes #4603

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Documentation build overview

📚 dev | 🛠️ Build #30020849 | 📁 Comparing 7ff50b0 against latest (9d7adbd)

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File	Status
contribute.html	📝 modified
install.html	📝 modified
search-integration.html	📝 modified
style-guide.html	📝 modified
design/embed-api.html	📝 modified
design/theme-context.html	📝 modified

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Documentation build overview

📚 docs | 🛠️ Build #30020850 | 📁 Comparing 7ff50b0 against latest (9d7adbd)

🔍 Preview build

Show files changed (5 files in total): 📝 5 modified | ➕ 0 added | ➖ 0 deleted

File	Status
build-customization.html	📝 modified
support.html	📝 modified
about/index.html	📝 modified
reference/features.html	📝 modified
tutorial/index.html	📝 modified