Landing Page: Improve design

[t-hamano]

This small PR is to improve the design and UX of the landing page and includes three changes, and I would appreciate your feedback on whether each change makes sense.

Before

After

1. Remove custom link/button color

Respects the WordPress core link color. This color is also applied to link text in the block editor.

Before	After

2. Remove light font weight

I don't know what the intention of this style is, but the text is a little hard to read in my environment.

Before	After

3. Change cursor style for summary element

Indicates that the content can be opened or closed. This style also exists in the Details block.

[vcanales]

I think these changes are good.

cc. @beafialho for a designer's perspective

[mikachan]

These changes all look like good improvements to me, but happy to defer to @beafialho before bringing them in.

[beafialho]

Fine to adjust the font weight to Regular but the intention of the design is to use the same link color as the plugin color (#3858E9). This contributes to a more cohesive, pleasurable user experience of the plugin.

[t-hamano]

Thank you everyone for your reviews!

the intention of the design is to use the same link color as the plugin color (#3858E9).

I see, I've restored the link color.

[mikachan]

Looks good, thanks @t-hamano!