Table of contents fixes

[jblang]

First off, let me say the level of detail in your docs is amazing. The amount of time and effort you've put into researching these parts is clear. That said I think you could use an editor :).

This PR mainly fixes the TOC by replacing links to README.md#some-heading, with just #some-heading. Having README.md in the link causes the entire page to reload whenever you click a link, and totally breaks the links when viewing the repo on the iPad app, because it just reloads the page and doesn't jump to the heading.

In addition, I took the liberty of moving the announcements to a separate section below the TOC (but with links at the start of the TOC). Speaking as a new user of your repo, I initially found the README overwhelming, especially since I had to scroll through several pages talking about problems for which I had no context before even finding the TOC. I also put an abbreviated version of the "What is DxCore" section at the top. I removed the commentary about naming and history related to tiny 0/1-series, and made it "just the facts". I retained your full original text in the "What is DxCore" section, which is also linked in the TOC.

I hope you don't mind these changes. If you're receptive, I will probably do more PRs for some typos I've noticed as I've been reading the docs.