Fix CI lint failures and polish the documentation site (Material theme, logo, favicon)

A group of documentation and CI polish items, grouped because they all sit in the docs tooling and mkdocs.yml.

CI lint failures

The validate stage has two failing lint jobs. Both are allow_failure, so the pipeline reports green while they fail.

• cspell: two words are missing from the project dictionary, webforms (the plural form; webform is already allowed) and WSOD. Add both to .cspell-project-words.txt.
• ESLint: mkdocs.yml raises two yml/indent errors. The nav entries under "Cross-site API" are indented four spaces and should be six.

Documentation site theme

The documentation currently renders with the default readthedocs theme. Switch mkdocs to the Material theme (mkdocs-material) for a clearer, searchable docs site, and make sure the documentation build installs mkdocs-material.

Logo and favicon

Add a project logo and a favicon for the docs site (the Material theme logo and favicon options), with the image assets stored under the docs directory.

Proposed resolution

• Add webforms and WSOD to the cspell dictionary.
• Set the Material theme in mkdocs.yml (which also corrects the nav indentation) and ensure the build installs mkdocs-material.
• Add logo and favicon assets and reference them from mkdocs.yml.