Complete rework of the documentation Onboarding with new "Analytics Overview"

[Jeehut]

I know, this PR has a lot of changes, I'll do my best to explain the most notable differences down below. Before doing that, please note that we need to also merge the related PR on the website repo at the same time, which renames the sidebar entry titles and helps with link discovery (thankfully it's a small PR!): https://github.com/TelemetryDeck/website/pull/276.

Also related in web repo (to be merged together): https://github.com/TelemetryDeck/web/pull/2239.

1. This introduces a new section named "App Analytics Guide" with 4 new documentation pages:
   a. "Analytics Overview": This is the new go-to article that works as the hub for the most relevant articles for beginners.
   b. "Acquisition Analytics": A detailed explanation how to make the most of the new "Acquisition" insights.
   c. "Metrics Analytics": A detailed explanations how to make the most of the existing "Metrics" tab (all entries in one).
   d. "The Pirate Metrics Framework": The theory behind the AARRR stats, mostly explains why we're using it. Only for people interested.

2. The existing sidebar entries were renamed (without changing their file names to keep existing links intact) to be more clear in terms of what the purpose of each article is. See also the table below to compare the changes:
   a. "Introduction" -> "Getting Started"
   b. "Getting Started" -> "Quick Start Guide"
   c. "Guides" -> "SDK Setup Guides"
   d. "React and React Native Setup Guide" -> "React (Native) Setup Guide"
   e. "Articles" -> "Advanced Topics"

   Before	After

3. Each "Setup Guide" now ends with the following section, pointing to the "Analytics Overview" article as a next step:
   

   Each Mobile Setup Guide was also streamlined to explain how to verify if sending the first signal worked as expected. Also, the sending of custom signals no longer sounds like a "required" step but is now approached as an "advanced" step so developers can ship their app even faster and make use of built-in insights before moving on to custom ones. "Progressive Disclosure".

4. Here's the full "Analytics Overview" page as a web archive so you can open and read it in Safari with all images etc to review its contents easily – note that it links to 16 different doc pages and serves as the entry point to discover TelemetryDeck:
   App Analytics Overview.webarchive.zip

5. Likewise, I have attached the "Acquisition Analytics" article, which explains each stat and how to learn from it:
   Acquisition Analytics.webarchive.zip

6. Here's the new "Metrics Analytics" doc explaining the Metrics tab in detail:
   Metrics Analytics.webarchive.zip

7. And last but not least "The Pirate Metrics Framework" doc page
   The Pirate Metrics Framework.webarchive.zip

8. What was the "Getting Started" page previously (now the "Quick Start Guide") has been revised to better work with this new onboarding approach.

[kkostov]

I don't think we need special wording for test mode for Flutter as it's the same behaviour as the KotlinSDK and SwiftSDKs. This section can be replaced with the same one from the other two pages.

[Jeehut]

I'm sorry, but I'm confused about your comment here. How is this line related to test mode? I see that it has "Manual Test Mode control" down there, but are you suggesting to remove it? What do you mean by "same one from the other two pages"? I already have the same "Verify your setup" section from the other two pages. Did you mean that? I'm a bit confused. 🤔

[Jeehut]

Hmm, I think you were referring to your comment above for Android, so I changed the section above (not the commented line) to:

{% notewarning "When running from your IDE, you're sending test signals" %}
If your app is configured with a debug build type (i.e. the default build variant running from Android Studio), your signals will be tagged as **Test Signals**, meaning that you can easily filter them out later. You'll see them show up in the TelemetryDeck Dashboard when the **Test Mode** toggle under the tab bar is turned on.
{% endnotewarning %}