Update docs latest version to 7.6 (#1831)

## Summary
- Set DOCS_LATEST_VERSION to 7.6
- Add 7.5 to the version selector
- Update the legacy versions page

## Test plan
- Not run (config/navigation content change only)

Author: DaedalusG

docs.config.js

@@ -1,5 +1,5 @@
 const config = {
-	DOCS_LATEST_VERSION: '7.5'
+	DOCS_LATEST_VERSION: '7.6'
 };
 
 module.exports = config;

docs/legacy.mdx

@@ -6,6 +6,7 @@
 
 <Accordion title="Sourcegraph 7.X">
 
+- [7.5](https://7.5.sourcegraph.com)
 - [7.4](https://7.4.sourcegraph.com)
 - [7.3](https://7.3.sourcegraph.com)
 - [7.2](https://7.2.sourcegraph.com)

src/data/versions.ts

@@ -13,6 +13,10 @@ export const versions: VersionI[] = [
 		label: 'latest',
 		url: '/docs'
 	},
+	{
+		name: 'v7.5',
+		url: 'https://7.5.sourcegraph.com'
+	},
 	{
 		name: 'v7.4',
 		url: 'https://7.4.sourcegraph.com'