[docs][glossary] Add page that lists all standardized k8s terms (#5657)

* Add comprehensive glossary page

Additional Authors:
* Andrew Chen <chenopis@users.noreply.github.com>
* Steve Perry <steveperry-53@users.noreply.github.com>

* incorporate Tim's feedback

* address zach's comments

* address more comments

* add additional crosslinking

* Add platform developer, tweak formatting for disambiguation cases or multiparagraph definitions.

_data/glossary/upstream.yaml

@@ -0,0 +1,11 @@
+id: upstream
+name: Upstream (disambiguation)
+related:
+- downstream
+tags:
+- community
+short-description: |
+  May refer to: core Kubernetes or the source repo from which a repo was forked.
+long-description: |
+  * In the **Kubernetes Community**: Conversations often use *upstream* to mean the core Kubernetes codebase, which the general ecosystem, other code, or third-party tools relies upon. For example, [community members](#term-member) may suggest that a feature is moved upstream so that it is in the core codebase instead of in a plugin or third-party tool.
+  * In **GitHub** or **git**: The convention is to refer to a source repo as *upstream*, whereas the forked repo is considered *downstream*.