Fix link issues after hard-reloading pages

[planger]

On hard reload the URL ending with xyz/ sometimes becomes xyz, which then breaks all relative internal links. With this change, we are adding gatsby-plugin-catch-links to ensure internal links work either way. Also we update the readme to reflect where the website is now hosted.

[github-actions]

PR Preview Action v1.4.7
Preview removed because the pull request was closed.
2024-04-19 09:26 UTC

[xai]

I could reproduce the problem on current master with, .e.g., https://theia-ide.org/docs/services_and_contributions/ and its link to theia-extensions: Hard reload -> click -> 404.

After testing several pages (and, additionally, using a link-checking tool), I can confirm that this behavior is gone in the preview of this PR. According to git grep -Eo '\[.+\]\(.*\)' | grep -Ev '\((/|http)', we only have very few relative links left:

src/docs/authoring_vscode_extensions.md:[prerequisites](#prerequisites-for-running-vs-code-extensions-in-theia))
src/docs/blueprint_documentation.md:[Building a product and installers](#building-a-product-and-installers)
src/docs/blueprint_documentation.md:[Signing](#signing)
src/docs/blueprint_documentation.md:[Adding/removing Features](#addingremoving-features)
src/docs/blueprint_documentation.md:[Updating Bundled VS Code Extensions](#updating-bundled-vs-code-extensions)
src/docs/blueprint_documentation.md:[Customizing Theia Extensions](#customizing-theia-extensions)
src/docs/blueprint_documentation.md:[Branding](#branding)
src/docs/blueprint_documentation.md:[Configure Publish and Update](#configure-publish-and-update)
src/docs/summary.md:[Theia](README.md)
src/docs/summary.md:[Overview](Architecture.md)
src/docs/summary.md:[Extensions](Extensions.md)
src/docs/summary.md:[Services and Contributions](Services_and_Contributions.md)
src/docs/summary.md:[Build your own IDE](Composing_Applications.md)
src/docs/summary.md:[Authoring an Extension](Authoring_Extensions.md)
src/docs/summary.md:[Authoring Plug-ins](Authoring_Plugins.md)
src/docs/summary.md:[Commands and Keybindings](Commands_Keybindings.md)
src/docs/summary.md:[Preferences](Preferences.md)
src/docs/summary.md:[Events](Events.md)
src/docs/summary.md:[Communication via JSON-RPC](Json_Rpc.md)
src/docs/summary.md:[Download](blueprint_download.md)
src/docs/summary.md:[Documentation](blueprint_documentation.md)

The ones in blueprint_documentation would not cause issues, the ones in summary.md do but I cannot find where that site is linked/used.