Add guide for mono space fonts

[davelab6]

@sajatype @colinmford In monospaced fonts, we make the combining marks the same width as all the other glyphs, and then collapse their widths to zero in GPOS as a first step in the mark feature. https://fosstodon.org/@tiro_j/110307327302489293

There's other mono space dogma in font bakery, would be good to preempt those checks by adding the lore to the guide

[eliheuer]

I can work on this when I get back to working on Hasubi Mono.

[vv-monsalve]

We have specific requirements for

• Monospace fonts
• Combining marks after the Anchors subtitle. We could turn those requirements into a subhead on its own so it gets added to the Table of Contents, though.

[vv-monsalve]

• We could turn those requirements into a subhead on its own so it gets added to the Table of Contents, though.

Made in #150