minor symfony#2992 [Icons][UX Icons] Update the links to some icon sets (javiereguiluz)

This PR was merged into the 2.x branch.

Discussion
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[Icons][UX Icons] Update the links to some icon sets

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Docs?         | yes
| Issues        | -
| License       | MIT

All the other icon sets are linked to their GitHub repos, so we can do the same for Bootstrap Icons and Boxicons.

Commits
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3f0d7f2 [UX Icons] Update the links to some icon sets

Author: Kocal

src/Icons/doc/index.rst

@@ -656,8 +656,8 @@ Learn more
 .. _`symfony/asset-mapper`: https://symfony.com/doc/current/frontend/asset_mapper.html
 .. _`symfony/ux-twig-component`: https://symfony.com/bundles/ux-twig-component/current/index.html
 .. _`W3C guide about SVG icon accessibility`: https://design-system.w3.org/styles/svg-icons.html#svg-accessibility
-.. _`Bootstrap Icons`: https://icons.getbootstrap.com/
-.. _`Boxicons`: https://boxicons.com/
+.. _`Bootstrap Icons`: https://github.com/twbs/icons
+.. _`Boxicons`: https://github.com/atisawd/boxicons
 .. _`Flowbite`: https://github.com/themesberg/flowbite
 .. _`FontAwesome Icons`: https://github.com/FortAwesome/Font-Awesome
 .. _`Heroicons`: https://github.com/tailwindlabs/heroicons