docs: honor image height and size unsized images by resolution

[grandixximo]

Fixes #4201.

Problem

Two related image-sizing problems in the docs build, both reported in #4201:

1. height is ignored. image::foo.png[height=400] had no effect. In HTML the bundled asciidoctor stylesheet's img{height:auto} (an author rule) overrides the HTML height attribute; in PDF asciidoctor-pdf sizes raster images only by the width family and drops height entirely. So both backends rendered Figures 1 and 4 of motion/5-axis-kinematics.adoc at full size instead of the requested 400px.

2. Unsized images are blown up. apply_default_width forced every width/height-less image to 75% of the text column. That stretched small images far beyond their intent: the 96-DPI equation captures rendered about 4x too large, and screenshots such as the pyvcp dial (a 210px window) rendered at roughly 40 DPI / 5.2in.

Changes

• Honor height in HTML. A postprocessor mirrors the width/height attributes into an inline style, which wins the cascade over the stylesheet. Bare numbers become px; values with a unit or % pass through.
• Honor height in PDF. When only a height is given, convert it to the equivalent pdfwidth using the image's intrinsic aspect ratio (PNG/JPEG/SVG size read without an image library).
• Size unsized images by resolution. Instead of a blanket 75%, size each image at its intended resolution: the embedded PNG pHYs when present, else an assumed 96 DPI screen capture. Images wider than the 75% cap, or whose size cannot be read, keep the 75% fallback.
• Build dependency. The HTML and PDF rules did not depend on docs/src/extensions/*.rb, so editing a resolver left previously built docs stale. Added DOC_EXTENSIONS as a prerequisite of both rules.

Verification

Full make htmldocs pdfdocs build, no errors. Confirmed in the rendered output:

• HTML: Figures 1 and 4 now carry style="height:400px".
• PDF (measured via the image transform matrix): Figure 1 (792x612) and Figure 4 (612x792, portrait) both render 300pt tall; the no-height figure stays at the 75% default.
• Equations now render at their 96-DPI size (e.g. equation__38, 130px, drops from 374pt to 97pt).
• The pyvcp dial drops from 374pt to 157pt (~2.2in), matching its on-screen size.

Note for reviewers

The resolution-based sizing affects many metadata-less screenshots across the docs: any raster narrower than about 498px now renders at its native 96-DPI size rather than 75% of the column; wider images are unchanged (still capped at 75%). This is a broad but intentional visual change. I spot-checked GUI/virtual-panel pages and the results look correct, but it is worth a look across chapters.