Split and Organise documentation files (#404)

* Set global TOC collapse to false

* Split documentation

I tried to split the index.md file into logical parts, not changing the contents.
I did add a space and change one HTTP URL to HTTPS.

---------

Co-authored-by: Sean Whalen <44679+seanthegeek@users.noreply.github.com>

docs/source/api.md

@@ -0,0 +1,34 @@
+# API reference
+
+## parsedmarc
+
+```{eval-rst}
+.. automodule:: parsedmarc
+   :members:
+```
+
+## parsedmarc.elastic
+
+```{eval-rst}
+.. automodule:: parsedmarc.elastic
+   :members:
+```
+
+## parsedmarc.splunk
+
+```{eval-rst}
+.. automodule:: parsedmarc.splunk
+   :members:
+```
+
+## parsedmarc.utils
+
+```{eval-rst}
+.. automodule:: parsedmarc.utils
+   :members:
+```
+
+## Indices and tables
+
+- {ref}`genindex`
+- {ref}`modindex`