Add back links

docs/backend-development.md

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+[⬅ Back to Index](./index.md)
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 # Developing Zed's Backend
 
 Zed's backend consists of the following components:

docs/building-zed.md

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+[⬅ Back to Index](./index.md)
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 # Building Zed
 
 How to build Zed from source for the first time.

docs/company-and-vision.md

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+[⬅ Back to Index](./index.md)
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 # Company & Vision
 
 ## Vision

docs/design-tools.md

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+[⬅ Back to Index](./index.md)
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 # Design Tools & Links
 
 Generally useful tools and resources for design.

docs/index.md

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+[⬅ Back to Index](./index.md)
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 # Welcome to Zed
 
 Welcome! These internal docs are a work in progress. You can contribute to them by submitting a PR directly!

docs/release-process.md

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+[⬅ Back to Index](./index.md)
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 # Zed's Release Process
 
 The process to create and ship a Zed release

docs/tools.md

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+[⬅ Back to Index](./index.md)
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 # Tools
 
 Tools to get started at Zed