[docs] Update CONTRIBUTING.md, add pull request templates (#1216)

* [docs] Update contributing.md

- Add Pull Request process and guidelines.
- Add feature/bug issue process.
- Rearrange some sections for clarity.
- Add overview of package structure.

* [docs] Add build from source links

* [chore] add pull request templates

These link to the new CONTRIBUTING.md document, and include a checklist to validate that contributors have read the guidelines.

* [docs] Put existing stub CoC in separate doc

* update web related stuff in CONTRIBUTING.md

Co-authored-by: f0x <f0x@cthu.lu>

.github/PULL_REQUEST_TEMPLATE/bugfix.md

@@ -0,0 +1,15 @@
+# Description
+
+> Please include a summary of the change and which issue is fixed.
+
+Fixes # (issue)
+
+# Checklist:
+
+- [ ] I have read the [GoToSocial contribution guidelines](https://github.com/superseriousbusiness/gotosocial/blob/main/CONTRIBUTING.md).
+- [ ] I have performed a self-review of my own code.
+- [ ] I have commented my code, particularly in hard-to-understand areas.
+- [ ] I have made any necessary changes to documentation.
+- [ ] New and existing tests pass locally with my changes.
+- [ ] I have run `go fmt ./...` and `golangci-lint run`.
+- [ ] I have added tests that prove my fix is effective.

.github/PULL_REQUEST_TEMPLATE/documentation.md

@@ -0,0 +1,3 @@
+# Description
+
+> Please briefly explain the documentation change that you've made, and why.

.github/PULL_REQUEST_TEMPLATE/feature.md

@@ -0,0 +1,16 @@
+# Description
+
+> Please include a summary of the feature you've coded, and link to the issue that it closes/implements.
+
+Implements # (issue)
+
+# Checklist:
+
+- [ ] I have read the [GoToSocial contribution guidelines](https://github.com/superseriousbusiness/gotosocial/blob/main/CONTRIBUTING.md).
+- [ ] I have discussed my implementation already, either in an issue on the repository, or in the Matrix chat.
+- [ ] I have performed a self-review of my own code.
+- [ ] I have commented my code, particularly in hard-to-understand areas.
+- [ ] I have made any necessary changes to documentation.
+- [ ] New and existing tests pass locally with my changes.
+- [ ] I have run `go fmt ./...` and `golangci-lint run`.
+- [ ] I have added tests that cover my new code.