Page Header - Increase Text Contrast

# What
On the landing page (https://parquet.apache.org/) increase the text contrast for:

- the navigation bar
- the summary about what Parquet is in the middle

The affected areas are marked on this screenshot:

# Why
Now I could make a subjective argument and say "look at it, it feels hard to read" and if you agree with that you can skip the remaining _Why_ section.

But there's also a strictly technical argument here: Accessibility/A11y as defined by the W3C, specifically [WCAG 2.2 - Technique G18](https://www.w3.org/WAI/WCAG22/Techniques/general/G18). WCAG is the underlying technical specification for many laws around the world ([reference](https://en.wikipedia.org/wiki/Web_Content_Accessibility_Guidelines#WCAG_referenced_by_law)), which I think is a solid reason why we should at least try to adhere to it.

# How
There are multiple ways of fixing this. I personally think that one will always have a hard time when trying to get a readable text overlaying a picture. Hence I suggest that the two sections mentioned above shall get a solid-colored background. That also makes the constrast calculation way easier if one wants to check if the contrast is "good enough".

Start by locating the landing-page source for the navigation bar and middle summary, then inspect how their text is styled over the image. Done means both areas have improved readability through a solid-colored background or another approach that meets WCAG 2.2 Technique G18 contrast guidance; verify the rendered landing page.

Written by the indexing model from the issue text.