Fix all markdownlint errors across the site

[bbatsov]

I promised a while back that I'd take care of the remaining lint issues across the site, and here it is — this PR brings us from 938 markdownlint errors down to 0.

Most of the errors (840) were MD013 (line-length), which I've simply disabled — enforcing a line length limit on content-heavy markdown files creates more churn than value. I also disabled MD001 (heading-increment), since our Jekyll layouts provide the h1 and content pages intentionally start at h2/h3.

On the config side I also standardized on atx headings (no closing #) and fenced code blocks (instead of indented), which are the modern defaults most people expect.

The actual content fixes across ~15 files are all minor and non-breaking:

• Escaped trailing # in headings — headings ending in F# were misdetected as atx_closed style (e.g. ### Low risk ways to use F# → ### Low risk ways to use F\#)
• Converted indented code blocks to fenced in the install guides and README
• Normalized list marker spacing (1. → 1. ) in the install guides
• Added missing alt text to a handful of <img> tags
• Replaced generic link text like [here] and [(link)] with something descriptive (e.g. [on GitHub], [entire analysis])
• A few other small things: trailing whitespace, a missing final newline, emphasis used as a heading, and a couple of broken link fragments in teaching/research.md

The linter now reports 0 errors across all 150 markdown files:

markdownlint-cli2 v0.20.0 (markdownlint v0.40.0)
Linting: 150 file(s)
Summary: 0 error(s)