Post 2.26.0 Release Tasks

Umbrella Jira for updates to the release guide based on [~lostluck]'s experience releasing 2.26.0.
- Improve the formatting consistency of the release documentation
##  Formatting of the markdown version (eg. WRT linebreaks).
## Improve readability of sections through bolding highlighting.
## Grammar errors and typos.
- Improve the getting started tasks (be explicit on installed dependencies up front. and other environment assumptions).
- Make the use of arbitrary version numbers consistent / Glossary of SCRIPT_SUBSTITUTIONS at the top.
- Describe the "big picture" of a release to get the manager better situated.

Anything else while I'm editing that can help.

Imported from Jira [BEAM-11693](https://issues.apache.org/jira/browse/BEAM-11693). Original Jira may contain additional context.
Reported by: lostluck.