docs(contrib): Start guidelines for schema design (#15037)

### What does this PR try to resolve?

This was inspired by a recent Cargo team discussion on whether we should
generally elide default values.
This will also help with
https://rust-lang.github.io/rust-project-goals/2025h1/cargo-plumbing.html

Case studies in schema design:
- #14506
- #10543

### How should we test and review this PR?

### Additional information

src/doc/contrib/src/SUMMARY.md

@@ -14,6 +14,7 @@
     - [Architecture](./implementation/architecture.md)
     - [New packages](./implementation/packages.md)
     - [New subcommands](./implementation/subcommands.md)
+    - [Data Schemas](./implementation/schemas.md)
     - [Console Output](./implementation/console.md)
     - [Filesystem](./implementation/filesystem.md)
     - [Formatting](./implementation/formatting.md)

src/doc/contrib/src/implementation/schemas.md

@@ -0,0 +1,47 @@
+# Data Schemas
+
+Cargo reads and writes user and machine facing data formats, like
+- `Cargo.toml`, read and written on `cargo package`
+- `Cargo.lock`, read and written
+- `.cargo/config.toml`, read-only
+- `cargo metadata` output
+- `cargo build --message-format` output
+
+## Schema Design
+
+Generally,
+- Fields should be kebab case
+  - `#[serde(rename_all = "kebab-case")]` should be applied defensively
+- Fields should only be present when needed, saving space and parse time
+  - Also, we can always switch to always outputting the fields but its harder to stop outputting them
+  - `#[serde(skip_serializing_if = "Default::default")]` should be applied liberally
+- For output, prefer [jsonlines](https://jsonlines.org/) as it allows streaming output and flexibility to mix content (e.g. adding diagnostics to output that didn't prevously have it
+- `#[serde(deny_unknown_fields)]` should not be used to allow evolution of formats, including feature gating
+
+## Schema Evolution Strategies
+
+When changing a schema for data that is read, some options include:
+- Adding new fields is relatively safe
+  - If the field must not be ignored when present,
+    have a transition period where it is invalid to use on stable Cargo before stabilizing it or
+    error if its used before supported within the schema version
+    (e.g. `edition` requires a minimum `package.rust-version`, if present)
+- Adding new values to a field is relatively safe
+  - Unstable values should fail on stable Cargo
+- Version the structure and interpretation of the data (e.g. the `edition` field or `package.resolver` which has an `edition` fallback)
+
+Note: some formats that are read are also written back out
+(e.g. `cargo package` generating a `Cargo.toml` file)
+and those strategies need to be considered as well.
+
+When changing a schema for data that is written, some options include:
+- Add new fields if the presence can be ignored
+- Infer permission from the users use of the new schema (e.g. a new alias for an `enum` variant)
+- Version the structure and interpretation of the format
+  - Defaulting to the latest version with a warning that behavior may change (e.g. `cargo metadata --format-version`, `edition` in cargo script)
+  - Defaulting to the first version, eventually warning the user of the implicit stale behavior (e.g. `package.edition` in `Cargo.toml`)
+  - Without a default (e.g. `package.rust-version`, or a command-line flag like `--format-version`)
+
+Note: While `serde` makes it easy to support data formats that add new fields,
+new data types or supported values for a field are more difficult to future-proof
+against.