docs/website: Change highlight.js Theme

On both our main website and the API docs site, we should change the theme of the code highlighter. There are 2 issues we hope to solve:

* Make the code look more like it does in Visual Studio.
* Make the inline code blocks more visible. Currently, they look like disabled text.

It really strains your eyes to look at the current theme too long.

There is a blog post that specifically outlines the steps how to change the theme with DocFx:

https://www.cazzulino.com/customize-docfx.html

One theme I thought would be okay was the [VS theme here](https://highlightjs.org/static/demo/), but the one from the blog post (which I haven't previewed) might be closer to what we are looking for. But as long as the text looks less faded than it does now, it would be an improvement.

Note that the themes of DocFx are already exported to the [/websites/apidocs/Templates/DefaultTemplateNoAssets](https://github.com/apache/lucenenet/tree/Lucene.Net_4_8_0_beta00014/websites/apidocs/Templates/DefaultTemplateNoAssets) folder so they can be copied over to the "live" folders and modified easily.

Read the DocFx customization post, then inspect the exported themes in websites/apidocs/Templates/DefaultTemplateNoAssets and compare them with the theme assets used by the main website and API docs site. Choose a less faded, Visual Studio-like highlight.js theme and verify that code and inline code are more readable on both sites.

Written by the indexing model from the issue text.