docs: Split vim mode documentation into two pages, edit for clarity (#17614)

Closes #17215

Release Notes:

- N/A

---

This PR builds upon the vim mode documentation page and aims bring the
following improvements:

- Separate vim mode-specific configuration from introducing vim mode.
- Reformat some lists of provided commands and keymaps from code blocks
to sub-sections containing tables.
- Flesh out the text a little bit to make it more explicit in some
parts.
- Generally format notes and a couple of other things closer to some
other docs pages.

Checking the diff doesn't give a good idea of the changes, so here are
some before after images for quick examples of the kinds of changes
brought by this PR.

**Introducing the key differences of Zed's vim mode**

Before


![2024-09-09_22-12](https://github.com/user-attachments/assets/447418cb-a6e6-4f9c-8d4b-6d941126979e)

After


![2024-09-09_22-16](https://github.com/user-attachments/assets/be69f2d9-c3ae-4b34-978a-344130bee37c)

---

**Zed-specific vim key bindings**

Before


![2024-09-09_22-17](https://github.com/user-attachments/assets/88fdc512-a50b-487d-85d1-5988f15c2a6f)

After


![2024-09-09_22-18](https://github.com/user-attachments/assets/3b77c2f6-0ffa-4afc-a86d-1210ac706c8c)

docs/src/SUMMARY.md

@@ -17,7 +17,7 @@
 - [Snippets](./snippets.md)
 - [Themes](./themes.md)
 <!-- - [Fonts](./fonts.md) -->
-- [Vim](./vim.md)
+- [Vim Mode](./vim.md)
 
 # Using Zed