docs: Clarify Option + andThen guidance in pipeline docs

[maciejziemichod]

Type

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Description

This makes the guidance more consistent across docs and a little easier to apply in practice, especially for newcomers deciding between Option, Effect errors, and Exit.

It also aligns the pipeline docs with the Either docs, which already discourage using Either as an error type in Effect workflows. The previous building-pipelines example still modeled this pattern, so this update removes that inconsistency.

• Clarify the Effect.andThen section in building-pipelines to focus on Option interop.
• Replace the previous Either example with guidance on when Either is still useful at boundaries.
• Fix wording in option.mdx to consistently describe mixing Option with Effect.

Related

• Related Issue #
• Closes #

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