Automatic robots.txt / webmanifest generation

[MelihDarcanxyz]

What kind of changes does this PR include?

• Automatic robots.txt generation
• Automatic webmanifest / icon generation

Description

• Since a change of domain can result in many manual edits, it can increase the risk of wrong domains in some files. This is especially important for a template theme. So robots.txt is generated using astro.config.site now.
• User can change the icon.svg and forget about icons now. It's automatically handled by astro-webmanifest. Multiple sizes are generated including Apple touch icons.
• Also, webmanifest config takes it inputs largely from siteConfig which makes it robust to the manual edits. Change the site config and you're ready.
• Nothing visual has changed.

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[chrismwilliams]

Thanks @MelihDarcanxyz for doing this. I've added back some static links just to speed-up builds, particularly page heavy sites, and to show a favicon in dev