Document the icon type of web apps and extensions

[dschmidt]

Documents the Icon type introduced in opencloud-eu/web#3580: an icon name, a named icon with fillType and color, or an image icon with src and srcDark.

• New "Icons" section in the extensions docs.
• The extension type pages and the viewer/editor apps page use icon: Icon and link to that section.
• The sibling properties iconFillType, iconColor and fillType are described as deprecated.

Should only be merged after the web PR (is released?)

[Copilot]

🟡 Changes recommended

Two interface snippets omit deprecated compatibility fields retained by the linked upstream change.

2 open findings

• Retain deprecated optional fillType for compatibility · New
• Retain and deprecate optional iconFillType · New

What changed in this PR

Documents the new design-system Icon type across web apps and extensions.

Changes:

• Adds guidance for named and image icons.
• Updates extension interfaces and examples to use Icon.
• Notes deprecated sibling icon properties.

File	Description
viewer-editor-apps/​index.md	Updates file-extension icons.
extensions/​index.md	Adds shared icon documentation.
right-sidebar-panel-extensions.md	Updates sidebar panel icons.
resource-indicator-extensions.md	Updates indicator icons and example.
left-sidebar-menu-item-extensions.md	Links to icon guidance.
folder-view-extensions.md	Links to icon guidance.
floating-action-button-extensions.md	Updates button icon type.
app-menu-item-extensions.md	Updates menu-item icon type.
action-extensions.md	Documents dynamic Icon values.
account-extensions.md	Updates account extension icons.

🧠 Review effort: Balanced

---

Give feedback about Copilot approvals in this survey to enter a drawing for a $150 gift card.

[dschmidt]

Talked to @JammingBen: we both disagree with CoPilot here. We find it's confusing to have that in the docs.
The docs should reflect the current best practices. If developers need more detailed information on their old code, they can checkout actual types or old versions of the docs.

[JammingBen]

LGTM 👍 Marked as draft until after the upcoming rolling release has been released.