DOC-5253, DOC-5254, DOC-5268, DOC-5259 Clean up Phase 2, Phase 4, and the Phase 1 intro

[aimurphy]

The goal of this PR was to address some duplicate content, and to make sure all the ZDM task pages connect to their preceding and following pages. For example, Phase 2 refers back to Phase 1 and connects "next steps" to Phase 3, Phase 3 refers back to Phase 2 and looks ahead to Phase 4, and so on.
This is to help users follow the migration process through the phases and understand the relationship of the phases.

• Flattened the left nav for easier browsing: there are no "Phase" categories with only one item.
• Removed duplicate CDM, DSBulk Migrator, and Sideloader content in favor of the one page introducing the options for Phase 2 (This page is largely unchanged, except for a few lines under Sideloader) and the tool-specific information exclusively in each tool's section of the migration docs. This is how it was before but I removed duplicate content that 1) was inconsistently duplicated and 2) didn't really need to be duplicated). I also eliminated some partials that are no longer needed due to this change.
• Added some information to the end of Phase 3 about failed requests and the importance of the phase.
• Reorganized the DSBulk Migrator page for better flow. Fixed the examples to use token and application tokens for Astra instead of arbitrary username and password strings.
• Cleaned Phase 4.
• Revised the Phase 1 overview to separate the required vs optional pages.

[mendonk]

Some typos but that's all. Nicely done.

[plpesvc-ds]

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