Simple `icon` doesn't pass YAML validation

Bug description

The docs suggest you can supply a path directly to logo:

logo: posit.png

But it fails YAML validation

Steps to reproduce

---
title: "Untitled"
format: html
brand:
  logo: "posit.png"
---

Expected behavior

Logo should be found and used.

Actual behavior

Validation of YAML front matter failed.
In file simple-logo.qmd
(line 5, columns 9--19) Field "logo" has value "posit.png", which must instead be an object
4: brand:
5:   logo: "posit.png"
          ~~~~~~~~~~~
6: ---
✖ The value "posit.png" is of type string.
ℹ The error happened in location brand:logo.

Render failed due to invalid YAML.

Stack trace:
    at renderFileInternal (file:///Users/charlottewickham/Documents/posit/quarto-cli/src/command/render/render-files.ts:581:19)
    at eventLoopTick (ext:core/01_core.js:175:7)
    at async renderFiles (file:///Users/charlottewickham/Documents/posit/quarto-cli/src/command/render/render-files.ts:325:9)
    at async render (file:///Users/charlottewickham/Documents/posit/quarto-cli/src/command/render/render-shared.ts:102:18)
    at async renderForPreview (file:///Users/charlottewickham/Documents/posit/quarto-cli/src/command/preview/preview.ts:428:24)
    at async render (file:///Users/charlottewickham/Documents/posit/quarto-cli/src/command/preview/preview.ts:172:22)

Your environment

• OS: Mac 15.0

Quarto check output

quarto check
Quarto 99.9.9
[✓] Checking environment information...
      Quarto cache location: /Users/charlottewickham/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.4.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.46.3: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      commit: 19473984b54e37e97888a74a46fe58f68628899b
      Path: /Users/charlottewickham/Documents/posit/quarto-cli/package/dist/bin

[✓] Checking tools....................OK
      TinyTeX: v2024.03.13
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/charlottewickham/Library/TinyTeX/bin/universal-darwin
      Version: 2024

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.12.1
      Path: /Users/charlottewickham/Documents/posit/quarto-web/.venv/bin/python
      Jupyter: 5.7.2
      Kernels: julia-1.10, ark, python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.3.3
      Path: /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources
      LibPaths:
        - /Users/charlottewickham/Library/R/arm64/4.3/library
        - /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/library
      knitr: 1.45
      rmarkdown: 2.28

[✓] Checking Knitr engine render......OK