Consolidate/reorganize introductory concepts

[mDuo13]

The Introduction section of the concepts has gotten narrowed down to just 3 pages, all of which kind of need work:

• Introduction to Consensus: Maybe not the best introductory article, should reconsider where it fits into the journey of someone learning about XRPL for the first time (Is there other stuff they need to know first? Can we trim this down more?)
• XRP: Somewhat redundant with the XRP Overview and history pages.
• Software Ecosystem: Outdated, with links to deprecated services like the Data API and XRP-API, pages that have been moved/merged ("business, exchanges, and wallets"), and a lot of its purposes have been superseded by the Client Libraries page.

Meanwhile, there's no longer a technical introduction to the XRPL (#1202) to anchor this section.

We should probably do away with "Introduction" as a section. Perhaps "Payment System Basics" can be further split into a "Fundamentals" section that fulfills the original intent of "Introduction"—providing basic knowledge that's a prerequisite to understanding deeper features—and some other section that holds more nitty-gritty details that you may or may not need.

We should also provide a recommended reading order for the introductory docs so that people who prefer to do so can sit down and learn about things in a linear order that makes sense. Not everyone likes to learn this way, but it's useful for some people.