docs: document the font option of table cells

[lbesecker195]

What kind of change does this PR introduce?

Documentation for #1737.

The table docs list the cell font option only as "Font options for the cell". In #1737, font: { family: 'FAR' } with a font registered as FAR rendered in Helvetica. font: { src: 'FAR' }, suggested in the issue, worked, and the thread confirmed the keys aren't documented anywhere yet.

docs/table.md now describes the keys under "Cell options":

• src: a registered font name, a standard font name, or a font file path or buffer, like the first argument of doc.font().
• family: the font inside a collection or variable font given in src, like the second argument of doc.font(). It only applies when a src is set on the cell, its row, or its column, so a registered font name belongs in src.
• size: the font size.

After the option list, a short example uses a registered font by name, and one sentence explains how cell, row and column fonts combine. That matches the resolution in normalizeCell from #1770: a more specific src resets family, while family or size on their own refine what is inherited.

To check the text, I rendered a one-cell table for each form and recorded the doc.font() calls and the font used for the text:

cell font	font used
{ src: 'Rob' } with registerFont('Rob', 'tests/fonts/Roboto-Regular.ttf')	Roboto-Regular
{ family: 'Rob' } with the same registration	Helvetica, and doc.font() is never called with Rob
{ src: 'tests/fonts/Roboto-Medium.ttf' }	Roboto-Medium
{ src: 'Courier-Bold' }	Courier-Bold

The new code block comes after the last ![n]() example in table.md, and both docs generators reset their code block list per file, so the generated example images keep their indices.

Checklist:

• Unit Tests N/A
• Documentation
• Update CHANGELOG.md N/A
• Ready to be merged

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