WIP: Add README docs

[jxnblk]

• Rendered version: https://github.com/gatsbyjs/gatsby-starter-landing-page/blob/update-readme/README.md
• Theme README: https://github.com/gatsbyjs/gatsby-starter-landing-page/blob/update-readme/gatsby-theme-landing-page/README.md

Other unrelated changes:

• Added exports to gatsby-theme-landing-page/index.js for MarkdownText and Link; I think this is a good way to expose a more public API for helper components/hooks/etc that someone could use when shadowing or extending the theme. I didn't add everything to this, but put a couple in as as example
• Added .gitkeep to the site's src/gatsby-theme-landing-page shadow directory to make it easier and more foolproof to shadow components in the theme; we should consider what sort of placeholder shadow files we want to include in the final starter

[gatsby-cloud[bot]]

Gatsby Cloud Build Report

gatsby-starter-landing-page

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Build Details

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:clock1: Build time: 13s

Performance

Lighthouse report

Metric	Score
Performance	:green_heart: 100
Accessibility	:green_heart: 100
Best Practices	:green_heart: 100
SEO	:green_heart: 91

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