SEO, performance & accessibility improvements

[KovJonas]

I analyzed the website in terms of SEO, and made a list of areas where we could improve it:

SEO

• robots.txt is invalid (actually we don't even have one), actually this is the biggest issue with the site right now
• There's no H1 tag, we should have one on every page
• We have a proper title and meta description on the page, but it's the same for all pages (/, /explorer, /tools, and /about)
• The links on the footer are a little bit small on mobile (not the text, but the "tap area" - so we could just apply a little bit padding on them, or something like that)

Performance

• Set an explicit width and height on image elements (and later on animations).
• We can optionally compress a little bit the images for better performance.
  Note: will apply these changes in the animations PR!

Accessibility

• We should add an alt name for the logo

Best practices

• Add rel="noopener" or rel="noreferrer" to external links (like the links to app.agora.space on the hero section, the links in the Future Plans section, and the links on the footer

Optional - Note: will add an analytics tool in a separate PR.

• Add Google Analytics & Microsoft Clarity or maybe Plausible (?) to the site, so we can analyze how users interact with our website