refactor docs; minimal content change

[zilto]

Refactored the documentation, focusing on cleaning up the navigation pane.

Changes

• flattened customizing-execution into graph-adapters and lifecycle-hooks
• renamed getting-started to get-started
• shortened formulations (e.g., Reference Documentation -> Reference, Overview of Concepts -> Concepts)
• Merged some pages
• Created learning-resources.md
• Created Integrations subheader

How I tested this

I compiled the docs and successfully ran it locally.

Notes

• While the site may work, this may break links pointing to the docs (e.g., tryhamilton, blogs)
• Two pages were left unindexed under the unorganized-docs folder

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[sweep-ai]

Apply Sweep Rules to your PR?

• Apply: All new business logic should have corresponding unit tests.
• Apply: Refactor large functions to be more modular.
• Apply: Add docstrings to all functions and file headers.

[skrawcz]

Just change substack -> blog.

[skrawcz]

This doesn't render properly as a link on the comparison page:

[Hamilton + Metaflow] example: https:/outerbounds/hamilton-metaflow

[skrawcz]

just those two changes I think, otherwise LGTM.