docs: overhaul readme

The main README has been completely overhauled:

- It is now more closely aligned with Pact JS and Pact Go
- It is much shorter, with usage documentation having been moved into
  `/docs`
- Reference to Pact Python `v3` has been added.

Signed-off-by: JP-Ellis <josh@jpellis.me>

Author: JP-Ellis

.markdownlint.yml

@@ -1,11 +1,21 @@
 default: true
+
+# Do not enforce line length
+line-length: false
+
+# Adjust list indentation for 4 spaces
 list-marker-space:
   ul_single: 3
   ul_multi: 3
   ol_single: 2
   ol_multi: 2
-line-length: false
 ul-indent:
   indent: 4
+
+# Require fenced code blocks
 code-block-style:
   style: fenced
+
+# Disable checking for reference links, as MkDocs generates additional ones that
+# are not visible to MarkdownLint.
+reference-links-images: false