Make visual adjustments to the home page, polish the UI so that it matches Docusaurus styling (#110)

Description

Update the website's UI so that it follows the design principles, and is consistent throughout the website. Get rid of duplicate styling for small devices in the custom.css file.

The /pricing page and blog sections have not been touched, as they are on separate issues/branches.

Related issue

#110 (closed)

Examples

The following screens are taken from https://v2.postgres.ai/

Before:

After: Added hover transition that darkens the button background, increase h1 font-size and reduce p font-size.

Before:

After: Consistent title color, reduce font-size.

Before:

After: Vertically center the content on mobile devices, reverse-column for better readability.

Before:

After: Match button size with text, polish padding.

Before:

After: Reduce blog header title.

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes and they have been reviewed OR there are no API changes

Closes #110 (closed)