Add guide about secrets management with the Vault (#321)

## What does this PR do?

Document the secrets Vault.  
Also add a page for the new `context.secrets` object exposed in plugin context

kuzzleio/kuzzle#1315

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### Other changes

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### Boyscout

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Author: Aschen

src/core/1/guides/essentials/secrets-vault/index.md

@@ -0,0 +1,96 @@
+---
+code: false
+type: page
+title: Secrets Vault
+order: 700
+---
+
+# Secrets Vault
+
+<SinceBadge version="1.8.0" />
+
+When you develop an application with Kuzzle, you may need to use secrets such as API keys or authentication information.  
+
+Of course, it is unacceptable to version these secrets in cleartext with the rest of your source code.  
+
+However, it is still practical to be able to share these secrets with the rest of your team, or to add them to the repository for automated production. 
+
+Kuzzle offers a secure storage system for these secrets, the operation is as follows:
+  - writing secrets to a JSON file,
+  - manual encryption of this file with a password,
+  - adding the encrypted file to the repository,
+  - automatic decryption of the file when Kuzzle starts,
+  - exposing the secrets in the context of plugins.
+
+Thus, the only secret that it is necessary to communicate to the rest of a team is the encryption password for this file.
+
+## Secrets file format
+
+The secrets file is in JSON format. String values are encrypted but the key names remain the same.
+
+```js
+/* config/secrets.json */
+{
+  "aws": {
+    "secretKeyId": "lfiduras"
+  },
+  "cloudinaryKey": "ho-chi-minh"
+}
+```
+
+Once encrypted, the file looks like the following:
+
+```js
+/* config/secrets.enc.json */
+{
+  "aws": {
+    "secretKeyId": "536553f3181ada6f700cac98100f1266.3181ada66536553f"
+  },
+  "cloudinaryKey": "f700cac98100f1266536553f3181ada6.6536553f3181ada"
+}
+```
+
+## Encrypt and decrypt with the CLI
+
+The encryption of a secret file is done using the CLI with the following command:
+
+```bash
+./bin/kuzzle encryptSecrets config/secrets.json --vault-key strongpassword
+[ℹ] Encrypting secrets...
+[✔] Secrets successfully encrypted: config/secrets.enc.json
+```
+
+The file `config/secrets.enc.json` can be added safely to the project repository.
+
+To decrypt a previously encrypted file, use the following command:
+
+```bash
+./bin/kuzzle decryptSecrets config/secrets.enc.json --vault-key strongpassword
+[ℹ] Decrypting secrets...
+[✔] Secrets successfully encrypted: config/secrets.json
+```
+
+::: info
+You can also specify the vault key in the `KUZZLE_VAULT_KEY` environment variable.
+:::
+
+## Load encrypted secrets at startup
+
+Kuzzle will try to decrypt the provided file using the following locations, in that order of priority:
+  - in the command line: `./bin/kuzzle start --secrets-file /var/secrets.enc.json`
+  - in an environment variable `export KUZZLE_SECRETS_FILE=/var/secrets.enc.json`
+  - the default one present at the location `<kuzzle dir>/config/secrets.enc.json`
+
+The decryption key must be provided in one of the following ways, in order of priority as well:
+  - in the command line: `./bin/kuzzle start --vault-key verystrongpassword`
+  - in an environment variable `export KUZZLE_VAULT_KEY=verystrongpassword`
+
+Kuzzle start sequence ends in failure if:
+  - a decryption key is provided and Kuzzle cannot find a file
+  - Kuzzle finds a file and no decryption key is provided
+
+## Accessing secrets in plugin
+
+Once Kuzzle has successfully loaded the file containing the secrets, it exposes its decrypted content to all plugins.  
+
+Secrets are accessible in the [secrets](/core/1/plugins/plugin-context/secrets) property of the plugin context.  

src/core/1/plugins/plugin-context/secrets/index.md

@@ -0,0 +1,33 @@
+---
+code: true
+type: page
+title: secrets
+---
+
+# secrets
+
+<SinceBadge version="1.8.0" />
+
+Secrets contained in the [Vault](/core/1/guides/essentials/secrets-vault) and loaded at Kuzzle startup.
+
+They have the same format as in the JSON file:
+
+```js
+/* config/secrets.enc.json */
+{
+  "aws": {
+    "keyId": "a47de7426fbcb8904290e376f147bc73.8e4b35be62ecbc53"
+  }
+}
+```
+
+```js
+init (config, context) {
+  console.log(context.secrets);
+  // {
+  //   aws: {
+  //     keyId: 'decrypted aws key id'
+  //   }
+  // }
+}
+```