add documentation guide for adding new secrets (#7072)

* docs: add guide for adding new secrets

* generalize docs for adding secrets/settinsg

Co-authored-by: repo-ranger[bot] <39074581+repo-ranger[bot]@users.noreply.github.com>

doc/adding-new-config-values.md

+# Adding New Config Values
+
+The Badge Server supports a [variety of methods for defining configuration settings and secrets](./server-secrets.md), and provides a framework for loading those values during bootstrapping.
+
+Any new configuration setting or secret must be correctly registered so that it will be loaded at startup along with the others.
+
+This generally includes adding the corresponding information for your new setting(s)/secret(s) to the following locations:
+
+- [core/server/server.js](https://github.com/badges/shields/blob/master/core/server/server.js) - Add the new values to the [schemas](https://github.com/badges/shields/blob/master/core/server/server.js#L118-L193). Secrets/tokens/etc. should go in the `privateConfigSchema` while non-secret configuration settings should go in the `publicConfigSchema`.
+- [config/custom-environment-variables.yml](https://github.com/badges/shields/blob/master/config/custom-environment-variables.yml)
+- [docs/server-secrets.md](https://github.com/badges/shields/blob/master/doc/server-secrets.md) (only applicable for secrets)
+- [config/default.yml](https://github.com/badges/shields/blob/master/config/default.yml) (optional)
+- Any other template config files (e.g. `config/local.template.yml`) (optional)
+
+The exact values needed will depend on what type of secret/setting you are adding, but for reference a few commits are included below which added secrets and or settings:
+
+- (secret) [8a9efb2fc99f97e78ab133c836ab1685803bf4df](https://github.com/badges/shields/commit/8a9efb2fc99f97e78ab133c836ab1685803bf4df)
+- (secret) [bd6f4ee1465d14a8f188c37823748a21b6a46762](https://github.com/badges/shields/commit/bd6f4ee1465d14a8f188c37823748a21b6a46762)
+- (secret) [0fd557d7bb623e3852c92cebac586d5f6d6d89d8](https://github.com/badges/shields/commit/0fd557d7bb623e3852c92cebac586d5f6d6d89d8)
+- (configuration setting) [b1fc4925928c061234e9492f3794c0797467e123](https://github.com/badges/shields/commit/b1fc4925928c061234e9492f3794c0797467e123)
+
+Don't hesitate to reach out if you're unsure of the exact values needed for your new secret/setting, or have any other questions. Feel free to post questions on your corresponding Issue/Pull Request, and/or ping us on the `contributing` channel on our Discord server.