Landing page updates/simplification

[rtaniwa]

/area documentation

/area landing-page

We need to do a refresh of the existing landing page content. This includes:

• Refresh with new copy for the Features section
• Remove the Why Devfiles section (redundant)
• Change the format of the value props section to show the headers, but only display the descriptive text on-demand
• Add a GitHub icon to the top of the page with a link to the devfile repo

[rtaniwa]

Going to close this out as a dupe of issue 859.