docs: improve installation instructions and customize theme

[frgfm]

This PR:

• updates the theme settings of the documentation
• updates quickstart and readme

Complete REL-194

[linear]

REL-194 Implements the new theme, including docs.json and associated assets for the documentation

Unfortunately, I was unable to push my branch to the Git repository.

I have prepared the docs.json file and new assets, including the light/dark logo and an Open Graph image, all contained in a .zip file.

I would appreciate it if we could replace the current green assets with these purple ones, as they align more closely with our landing page.
This change will promote consistency and reduce visual context switching for the user.

docs.zip

{
  "$schema": "https://mintlify.com/docs.json",
  "theme": "mint",
  "appearance": {
    "default": "dark",
    "strict": false
  },
  "name": "Relay",
  "colors": {
    "primary": "#746ADB",
    "light": "#8B82E6",
    "dark": "#5E52B0"
  },
  "favicon": "/favicon.svg",
  "navigation": {
    "tabs": [
      {
        "tab": "Documentation",
        "groups": [
          {
            "group": "Getting started",
            "pages": [
              "documentation/getting-started/introduction",
              "documentation/getting-started/quickstart",
              "documentation/getting-started/concepts"
            ]
          },
          {
            "group": "Community & support",
            "pages": [
              "documentation/community/roadmap",
              "documentation/community/contributing",
              "documentation/community/faq",
              "documentation/community/support"
            ]
          }
        ]
      },
      {
        "tab": "Changelog",
        "groups": [
          {
            "group": "Changelog",
            "pages": [
              "changelog/overview"
            ]
          }
        ]
      }
    ],
    "global": {
      "anchors": [
        {
          "anchor": "Documentation",
          "href": "https://docs.relaycli.com",
          "icon": "book-open-cover"
        },
        {
          "anchor": "Community",
          "href": "https://discord.gg/T4zbT7RcVy",
          "icon": "discord"
        },
        {
          "anchor": "Manifesto",
          "href": "https://relaycli.com/manifesto",
          "icon": "feather"
        }
      ]
    }
  },
  "logo": {
    "light": "/logo/black.svg",
    "dark": "/logo/white.svg",
    "href": "https://relaycli.com"
  },
  "navbar": {
    "links": [
      {
        "label": "Support",
        "href": "mailto:hi@relaycli.com"
      }
    ],
    "primary": {
      "type": "github",
      "href": "https://github.com/relaycli/relay"
    }
  },
  "footer": {
    "socials": {
      "x": "https://x.com/relaycli",
      "github": "https://github.com/relaycli",
      "linkedin": "https://linkedin.com/company/relaycli"
    }
  },
  "seo": {
    "metatags": {
      "og:title": "Relay",
      "og:description": "Email infrastructure for modern applications"
    },
    "image": "/meta/og.png"
  },
  "contextual": {
    "options": [
      "copy",
      "view",
      "chatgpt",
      "claude"
    ]
  },
  "fonts": {
    "family": "Geist Mono"
  }
}