Homepage: add overview of full node benefits

[harding]

Preview: http://dg1.dtrt.org/

This adds to the homepage a short one-paragraph overview of what a full node does. A button designed to look like a link can be clicked to display an additional four paragraphs about the benefits of Bitcoin Core. Two prominent links are emphasized pointing to pages I'd expect readers to find especially useful.

This is the first part of a series of PRs designed to copy most of the content from the old Bitcoin.org Bitcoin Core subsite to this website, updating it in the process. When all that content has been copied, two further changes are expected to this page:

1. The final paragraph of the extended description, which currently contains no links, will be updated to link various words to the corresponding feature pages.

2. The two icon-title links below the text (Download and Contribute) will be joined by additional links to Features and Get Help.

Two small commits in this PR affect other parts of the site:

• "CSS: change..." updates the default color for hyperlinks from black to blue. The black text on the existing site often looks to me like bolding rather than something you can click, and I think that needs to be fixed if we want to guide users to a larger set of available resources, like various pages with descriptions of Bitcoin Core's features.

• "Downloads: refactor..." takes code we have for finding the highest-number release on the site and makes it a function we can use on other pages, allowing us to mention the latest version of Bitcoin Core on the homepage. I could've done this as a move-only but the code, which I originally wrote, was dumb, so I made it less dumb (hopefully).

The new text will not appear on the translated homepages unless the translators update their strings and bump the version numbers.

[harding]

Thanks to @hebasto for poking me to put more resources on the website that educate users about the need for, and benefits of, economically using your own full nodes. Also many thanks to those who contributed to and helped review the original PR for this on Bitcoin.org almost six years ago: https://github.com/bitcoin-dot-org/Bitcoin.org/pull/1044

The next step in this series of PRs will be adding an updated version of the validation page (which I think I'll use this opportunity to rename to verification).

[hebasto]

Concept ACK.

[harding]

Realized this was accidentally based on an old branch and was showing 0.21.0 as the latest release; rebased to master with no code changes and confirmed it now correctly shows 0.21.1 as the latest release. Also updated the preview. Sorry for the confusion.

[katesalazar]

• [x] concept ack
• [ ] tested