Add new web extensions and extension points

[JammingBen]

Adds docs for the new extension FloatingActionButtonExtension and its extension point (introduced by opencloud-eu/web#1997) as well as a bunch of other extension points (introduced by opencloud-eu/web#2015) to the Web extension system dev docs.

[Copilot]

Pull request overview

Updates the Web extension system developer documentation to cover newly introduced extension types and extension points, including the new FloatingActionButtonExtension and additional header/sidebar extension point targets.

Changes:

• Document FloatingActionButtonExtension as a new predefined extension type and describe its behavior/usage.
• Expand the “Extension Points” section with newly introduced extension point IDs (header left/right, more files sidebar targets, floating action button mount).
• Add a dedicated extension-type doc page with configuration and an example for floating action button extensions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
docs/dev/web/extension-system/index.md	Adds the new extension type to the list and expands the catalog of supported extension points.
docs/dev/web/extension-system/extension-types/floating-action-button-extensions.md	New doc page describing the floating action button extension type, config interface, and example.

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