Convert RTC product guide to markdown

[asjohnston-asf]

Still a ways off, but wanted to at least but the draft in front of people and see if we like the styling.

• images are missing
• I took out the section numbers but I'm considering adding them back in Question posed in Should we include section numbers in the RTC product guide? #60
• I took out the abbreviations section but I'm considering putting it back (maybe on a separate page) Opened issue Include a glossary #59 to do this later
• I took out the change control page, I assume we'll let github handle that
• mkdocs only provides anchors and TOC entries for H2 and H3 headers, not H4, so there's a link to Quantifying Change over Time that's broken at the moment
• still need to fix the production document having two Figure 5s
• citations aren't quite formatted correctly (needs to indent on word-wrap) Opened issue Need better reference/citation formatting #61 to track

Also updated some out-of-date content: Opened #62 to track

• README.txt is now README.md.txt
• iso.xml is removed

[jhkennedy]

Our current mkdocs + material theme can handle a central glossary and hover over definitions of terms in the glossary:

https://squidfunk.github.io/mkdocs-material/reference/abbreviations/

[jhkennedy]

citations aren't quite formatted correctly (needs to indent on word-wrap)

It might be worth using mkdocs-bibtex to support citation management