docs: improve landing pages for Contrib Guide

Signed-off-by: Meg McRoberts <[email protected]>

docs/docs/contribute/index.md

@@ -4,10 +4,35 @@ comments: true
 
 # Contributing to Keptn
 
-This section is under development.
-Contents are not complete,
-but all material here has been reviewed for technical accuracy.
-
 Keptn software and documentation
 is developed and maintained by the Keptn community.
 We invite you to join us.
+This guide provides instructions and guidelines
+for contributing software, documentation, and tests to Keptn.
+
+Keptn software, documentation, and tests are stored in the same
+[Keptn](https://github.com/keptn/lifecycle-toolkit)
+GitHub repository.
+We use similar processes for all contributions.
+
+This guide provides useful information about how to contribute to the Keptn project.
+It is divided into three sections:
+
+* [General](general/index.md) includes information
+  that is relevant to all contributions, including
+  information you should know about the technologies used,
+  instructions for creating a PR that contains your contribution
+  by either forking and cloning the repo and creating contributions locally
+  or working in Github Codespaces,
+  and guidelines that apply to all contributions.
+
+* [Software contributions](software/index.md)
+  includes information about how to set up an environment
+  for developing and testing Keptn software
+  as well as specific instructions for creating a new
+  Keptn metrics provider.
+
+* [Documentation contributions](documentation/index.md)
+  gives additional guidelines that apply only to documenation contributions
+  and describes the documentation file structure
+  and the documentation tools we use.