feat: document group field dot notation in field add help

[angeloashmore]

Resolves: #139

Description

Documents the dot notation syntax for adding fields inside groups (e.g. prismic field add text my_group.description --to-type blog_post). Adds sections support to command routers so the field add router can show a GROUP FIELDS section in its --help output.

Checklist

• A comprehensive Linear ticket, providing sufficient context and details to facilitate the review of the PR, is linked to the PR.
• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

How to QA ¹

Run prismic field add --help and verify the GROUP FIELDS section appears explaining dot notation.

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Note

Low Risk
Low risk: changes only CLI help text rendering and adds an optional sections block to router help output, with no impact on command execution paths beyond printing help.

Overview
Adds optional sections support to createCommandRouter help generation so routers can display additional formatted documentation blocks.

Updates prismic field add --help to include a GROUP FIELDS section describing dot-notation (my_group.description) for adding fields inside existing groups.

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[angeloashmore]

Is it necessary to add that bit about group fields into every field type supported in group fields? Conversely, do we need to explicitly state in each field type if it is not supported in groups (for example, groups cannot have groups).

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[angeloashmore]

The dot notation section is only on the field add router help, not duplicated into each field type. This seems like the right place since it applies to all field types that support groups. I don't think we need to explicitly state in each field type whether it supports groups -- that's better left to the docs or error messages when someone tries to nest unsupported types.