Update the documentation of the website to highlight the new task runtimes

[RealAnna]

The Write KeptnTask page of our website should reflect the new possibility of defining tasks using other containers' runtime.

DoD

• Mention the limitation about a single runtime per task
• Write inside the KeptnTask page a section for each available runtime, namely container and Python
• Each section provides one or more tested examples
• Each section specifies from what version the runtime is available (aka container and python should be from 0.8 onward) - check if it makes sense based on the "version" drop-down menu

[StackScribe]

The "Write Keptn Tasks" page has been removed from the doc set. This information should be added to the following docs:

• KeptnTaskDefinition CRE YAML reference page -- this is where the information you list here needs to go
• Working with Keptn Tasks -- should be a "guide" to workng with Keptn tasks. Right now, it is kind of a catch-all for information that is related. It needs some rewrite and restructuring and we may end up breaking it up into smaller pages. But if we have information to impart that is not appropriate for the reference page, we can put it here or in its own page in the "Implementing..." section
• KeptnTaskDefinition API Reference Page -- yes, this is autogenerated but we shouldn't forget about it. I assume that this change warrants the creation of a v1alpha4 API version.

[RealAnna]

closed in favor of #1521 and #1579