chore: use json.Marshal for the message context

[zepatrik]

Related issue(s)

The docs is auto-generating with useless changes, see e.g. 64bf08d because we used sjson which does not order keys.
For less CI spam I am proposing to instead use json.Marshal. A quick benchmark shows that in this specific case sjson is only insignificantly faster:

Go benchmark

package sjsonbench_test

import (
	"encoding/json"
	"github.com/tidwall/sjson"
	"testing"
	"time"
)

var now = time.Now()

func BenchmarkSJSON(b *testing.B) {
	for i := 0; i < b.N; i++ {
		j := "{}"
		j, err := sjson.Set(j, "foobar", "some value")
		if err != nil {
			b.Errorf("error: %s", err)
		}
		j, err = sjson.Set(j, "asdf", 13241345)
		if err != nil {
			b.Errorf("error: %s", err)
		}
		j, err = sjson.Set(j, "jkhad", map[string]interface{}{"expired_at": now})
		if err != nil {
			b.Errorf("error: %s", err)
		}
	}
}

func BenchmarkGoJSON(b *testing.B) {
	for i := 0; i < b.N; i++ {
		_, err := json.Marshal(map[string]interface{}{
			"foobar": "some value",
			"asdf":   13241345,
			"jkhad":  map[string]interface{}{"expired_at": now},
		})
		if err != nil {
			b.Errorf("error: %s", err)
		}
	}
}

$ go test -bench .
goos: linux
goarch: amd64
pkg: sjsonbench
cpu: Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz
BenchmarkSJSON-4          295066              4535 ns/op
BenchmarkGoJSON-4         234032              5213 ns/op
PASS
ok      sjsonbench      3.522s
$ # Without the third key
$ go test -bench .
goos: linux
goarch: amd64
pkg: sjsonbench
cpu: Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz
BenchmarkSJSON-4          895790              1165 ns/op
BenchmarkGoJSON-4         675788              1684 ns/op
PASS
ok      sjsonbench      2.219s

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

Alternatively, we could also explicitly sort the keys before rendering the docs pages.