DRAFT docs: Revises quickstart with CLI-first approach

[vikram-dagger]

This commit revises the quickstart to use an example application and build a pipeline for it primarily by using the CLI, with additional examples of using custom Functions.

Also see #7275

[vikram-dagger]

This is a scratch PR to come up with a new approach and structure for the quickstart based on feedback to date.

The goals are:

• fewer or no "toy" or "hello world" examples - feedback is that users struggle to connect them with "real" CI pipeline operations
• explain reasons and benefits to create a custom module vs using one off Daggerverse - feedback is that we explain the "how" but not "why" or "when"
• quickstart steps should connect to each other - feedback is that while the examples on an individual page connect up, going from one page to another does not connect eg. there's no connection seen between the step of building a binary with golang and the step of building a container with wolfi (this connection exists but appears later, near the end of the quickstart, and does not resonate at the point of transition)
• less theory/concepts - this feedback is mainly related to the initial "hello world" examples and arguments, some users feel like these sections spend too much time on concepts
• actionable knowledge - feedback is that users are expecting the quickstart to teach them how to build a CI pipeline which they can apply to their own project, and it's not doing that

[vikram-dagger]

Closing in favour of #7275 per decision to try a code-first approach to the quickstart.