Convert "Tonc's Text Engine" to markdown (#73)

* Convert tte.htm to tte.md via pandoc.

This commit represents the automated portion of the conversion by
running

```
pandoc --from=html \
--to=markdown-fenced_divs-bracketed_spans-escaped_line_breaks-smart \
--wrap=none -o tte.md tte-old.htm
```

* Manual fixes for Tonc Text Engine page.

This commit fixes up the poritions of `tte.md` that can't auto-convert
via the `pandoc` command.

* Remove {target=\"_blank\"} instances from tte.md

A few were missed in the manual fixes to this file. Whoops!

* Update direct references to section numbers to links.

This commit removes section numbers that no longer exist from the
content, and instead now mentions the section title and additionally
links to the section.

content/SUMMARY.md

@@ -32,7 +32,7 @@
 - [Text systems](./text.md)
 - [Mode 7](./mode7.md)
 - [More Mode7 tricks](./mode7ex.md)
-- [Tonc's Text Engine]()
+- [Tonc's Text Engine](./tte.md)
 - [Whirlwind tour of ARM assembly](./asm.md)
 - [The Lab](./lab.md)