add robots.txt and sitemap to improve indexing

[seeReadCode]

we need better meta descriptions on pages too... we are using the site description on every page

c.f.

<meta itemprop="description" content="An online-first tech-enthusiast group serving folks around DFW interested in full stack development.">
...
<meta name="twitter:description" content="An online-first tech-enthusiast group serving folks around DFW interested in full stack development.">