docs(v1): add comprehensive application configuration reference

[shivansh-gohem]

What this PR does / Why we need it:
This PR provides the structured and updated v1 configuration reference for the Application configuration (app.config).

Note: This PR originally included the piped.config updates as well, but was split into two separate PRs per maintainer request to make reviewing safer and easier. The piped.config updates can now be found in #6644.

Key Documentation Updates:

• Documented the shift to the unified Application kind.
• Added the requirement for the pipecd.dev/platform label.
• Mapped all core v1 deployment specs including planner, pipeline, trigger, postSync, and driftDetection.

Which issue(s) this PR fixes:
Fixes #6542

[rahulshendre]

@shivansh-gohem, this is a good high‑level overview, but it only covers the top‑level fields plus DriftDetection.
Given the EPIC asks for “comprehensive configuration references”, I think we should either add
(a) document planner, pipeline, trigger, postSync, etc. here as well
or
(b) explicitly say that deeper details for these live in another doc (and link to it)

[shivansh-gohem]

Thanks for the review, @rahulshendre! That’s a very fair point. Since the Epic asks for comprehensive references, I completely agree with going with Option A. I have updated the PR to include the detailed markdown tables for Planner, Pipeline, Trigger, PostSync, NotificationRoute, and NotificationReceiver directly in the docs so everything is in one place!!!

[shivansh-gohem]

@eeshaanSA @khanhtc1202 could you ptal!!!

[khanhtc1202]

Lgtm with a small nit. Thank you!

[shivansh-gohem]

Thanks for the review, @khanhtc1202! Good catch on those two types. I've added the NotificationReceiverSlack and NotificationReceiverWebhook tables back into the document so the section is complete. Pushed the update!

[eeshaanSA]

I strongly feel, for a configuration reference, it is better to divide the app config reference and the piped config reference into 2 different PRs. These the the most important parts of the documentation, and doing them accurately is really important.

[shivansh-gohem]

Thanks for the feedback, @eeshaanSA! That makes complete sense. Since these are the two core configuration files, splitting them will definitely make reviewing and tracking changes much safer. I have updated this PR to exclusively cover the Application configuration, and I have opened a second, separate PR right now for the Piped configuration with my changes!!!
thank you!

[eeshaanSA]

@khanhtc1202, @Warashi we need some additional checks on this, since this the most important page in the document.

[eeshaanSA]

@shivansh-gohem, please focus on Application configuration for now. We can deal with the piped config later.