[docs] clearer structure for "combining blocks and allows" section (#4065)

# Description

This patch moves figures around in `admin/federation_modes.md` in order to make the text more approachable. Reading it, I found that I dwelled long on the chart, not thinking that there would be an easier to read explanation below, so I thought it'd make sense to swap the places. I also felt like the warning at the bottom was important enough that it'd better be up near the top of the section, highlighting the importance of understanding the explanation below.

I did not see a commit message style guide in project docs so I improvised based on recent commit messages

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [X] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [ ] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [X] I/we have not leveraged AI to create the proposed changes.
- [ ] I/we have performed a self-review of added code.
- [ ] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [ ] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4065
Co-authored-by: İ. Göktuğ Kayaalp <self@gkayaalp.com>
Co-committed-by: İ. Göktuğ Kayaalp <self@gkayaalp.com>

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