document merge options

[alexandratran]

Pull request checklist

Use the following template to make sure your PR fits the Teku documentation standard.

Before creating the PR

Make sure that:

• You've read the contribution guidelines.
• You've previewed your changes locally.

Describe the change

Document recently unhidden merge-related CLI options.

Issue fixed

fixes #348

Impacted parts

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• Documentation content
• Documentation page organization

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• CircleCI workflow
• Build and QA tools (for example lint or vale)
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Make sure that:

• You've fixed any issues raised by the tests.
• You've previewed your changes on Read the Docs
  and added a preview link.

Preview

see https://pegasys-teku--349.com.readthedocs.build/en/349/Reference/CLI/CLI-Syntax/#ee-endpoint

[tbenr]

LGTM with a suggestion. Feel free to improve wordings.

[rolandtyler]

lgtm

[bgravenorst]

Left some feedback. There are a few gaps that we will need to be address when fully documenting the merge feature.