Removed references to build_from_flow and block-based deployments for Prefect 3.0

[mthatt]

Removed mentions of build_from_flow everywhere except in ''Upgrade from Agents to Workers" guide. Removed 'Block and Agent-Based deployments" section.

Example

Checklist

• [ ] This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• [ ] If this pull request adds new functionality, it includes unit tests that cover the changes
• [x] This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.

For documentation changes:

• [x] This pull request includes redirect settings in netlify.toml for files that are removed or renamed.

For new functions or classes in the Python SDK:

• [ ] This pull request includes helpful docstrings.
• [ ] If a new Python file was added, this pull request contains a stub page in the Python SDK docs and an entry in mkdocs.yml navigation.