📝 Document nested config changes

Fixes

Related to https://github.com/FCP-INDI/C-PAC/pull/1411 by @shnizzedy Related to https://github.com/FCP-INDI/C-PAC/issues/1366 by @sgiavasis Related to https://github.com/FCP-INDI/fcp-indi.github.com/discussions/249 by @shnizzedy and https://github.com/FCP-INDI/fcp-indi.github.com/discussions/249#discussioncomment-339526 by @sgiavasis Includes #247

Description

These changes are live on my fork.

Documents the change to a nested config and lists the mappings from the old syntax to the new
Adds a warning to the "Design A Pipeline" section that alerts users to this change
Moves user pipeline-specific pages (1.7-1.8-nesting-mappings, default, design_a_pipeline, pipeline_config, and preconfig) from /user to user/pipelines
Replaces the tables built from docs/_sources/_static/params (like ) with relevant lines from the default pipeline config (like )

Screenshots

Checklist

[x] My pull request has a descriptive title (not a vague title like Update index.md).
[x] My pull request targets the source branch of the repository.
[x] My commit messages follow best practices.
[x] My code follows the established code style of the repository.
[ ] I added tests for the changes I made (if applicable).
[ ] I updated the changelog.
[x] I added or updated documentation (if applicable).
[x] I tried running the project locally and verified that there are no visible errors.

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FCP-INDI / fcp-indi.github.io