Missing documentation for the default image quality in xwiki.cfg and outdated default headless Chrome image in xwiki.properties

Two problems in the documentation of the configuration files:

• xwiki.plugin.image.defaultQuality is not documented in xwiki.cfg. It is the quality (between 0 and 1, default 0.5) used when the server resizes an image (e.g. an image inserted with a width) and no quality parameter is specified in the URL. It only applies to lossy formats such as JPEG. It's the main setting admins need to know about when resized images look bad (see XWIKI-19220), but it's only mentioned in Jira and on the forum. The "Image Plugin" section of xwiki.cfg only contains xwiki.plugin.image.cache.capacity.
• The default value of export.pdf.chromeDockerImage documented in xwiki.properties is zenika/alpine-chrome:latest, while the default image is femtopixel/google-chrome-headless (version 144.0.7524.3 at the moment) since XWIKI-23728. The Javadoc of PDFExportConfiguration#getChromeDockerImage() also mentions the latest tag, while a fixed version is used.