Improve detail about the Runtime Base Image

[lzdanski]

Issue Type

Inaccurate, misleading, or out-of-date information

Links to Affected Docs

https://docs.astronomer.io/astro/runtime-image-architecture#base-distributions

The Issue or Context

Document needs to explain what -base is and how the ONBUILD works.

Fix or Proposed Change

No response

Additional Notes

• https://astronomer.slack.com/archives/C015V2JFKT5/p1701449886188139
• https://www.notion.so/astronomerio/Doc-improvement-Runtime-561985f853184b42b033ab24ea729f2a?pvs=4

Required Reviewers

No response