docs: improve discoverability of build-secrets

[thaJeztah]

Description

• relates to dockerfile: how to build go project with private module? moby/buildkit#4791

While writing an answer to the discussion above, I noticed that the "build secrets" section is a bit hard to discover;

The "build overview" page has no tile for "secrets", so users will need to discover it from the "building" images navigation:

Neither the --mount type=secret, nor the docker build --secret flag cross-references the "build secrets" section.

I think we should improve that to help discoverability;

Tasks

Beta Give feedback

1. Add a "build secrets" or "working with secrets in your builds" tile.
2. Cross-reference the "build secrets" section from both the --mount type=secret, and docker build --secret sections.
3. Consider adding language-specific examples in the "getting started" sections (or elsewhere?), which, for Go, could include details about working with private modules (and provide links to Go's documentation on this as well).
4. Consider adding warnings to sections mentioned above about NOT using ARG for secrets (users are still tempting to do so).
5. :question: do we need some "do's / dont's" in the "security" section? (also see Secrets: write-up best practices, do's and don'ts, roadmap moby/moby#13490)

[thaJeztah]

cc @dvdksn