manual: Contributing -> Development, Hacking -> Building (#9014)

* manual: Contributing -> Development, Hacking -> Building

what's currently called "hacking" are really instructions for setting up
a development environment and compiling from source. we have
a contribution guide in the repo (which rightly focuses on GitHub
workflows), and the material in the manual is more about working
on the code itself.

since we'd otherwise have three headings that amount to "Building Nix",
this change also moves the "classic Nix" instructions to the top.

we may want to reorganise this in the future, and bring
contributor-oriented information closer to the code, but for now let's
stick to more accurate names to ease navigation.

doc/manual/src/development/contributing.md

@@ -0,0 +1,79 @@
+# Contributing
+
+## Add a release note
+
+`doc/manual/rl-next` contains release notes entries for all unreleased changes.
+
+User-visible changes should come with a release note.
+
+### Add an entry
+
+Here's what a complete entry looks like. The file name is not incorporated in the document.
+
+```
+---
+synopsis: Basically a title
+issues: 1234
+prs: 1238
+---
+
+Here's one or more paragraphs that describe the change.
+
+- It's markdown
+- Add references to the manual using @docroot@
+```
+
+Significant changes should add the following header, which moves them to the top.
+
+```
+significance: significant
+```
+
+<!-- Keep an eye on https://codeberg.org/fgaz/changelog-d/issues/1 -->
+See also the [format documentation](https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#changelog).
+
+### Build process
+
+Releases have a precomputed `rl-MAJOR.MINOR.md`, and no `rl-next.md`.
+
+## Branches
+
+- [`master`](https://github.com/NixOS/nix/commits/master)
+
+  The main development branch. All changes are approved and merged here.
+  When developing a change, create a branch based on the latest `master`.
+
+  Maintainers try to [keep it in a release-worthy state](#reverting).
+
+- [`maintenance-*.*`](https://github.com/NixOS/nix/branches/all?query=maintenance)
+
+  These branches are the subject of backports only, and are
+  also [kept](#reverting) in a release-worthy state.
+
+  See [`maintainers/backporting.md`](https://github.com/NixOS/nix/blob/master/maintainers/backporting.md)
+
+- [`latest-release`](https://github.com/NixOS/nix/tree/latest-release)
+
+  The latest patch release of the latest minor version.
+
+  See [`maintainers/release-process.md`](https://github.com/NixOS/nix/blob/master/maintainers/release-process.md)
+
+- [`backport-*-to-*`](https://github.com/NixOS/nix/branches/all?query=backport)
+
+  Generally branches created by the backport action.
+
+  See [`maintainers/backporting.md`](https://github.com/NixOS/nix/blob/master/maintainers/backporting.md)
+
+- [_other_](https://github.com/NixOS/nix/branches/all)
+
+  Branches that do not conform to the above patterns should be feature branches.
+
+## Reverting
+
+If a change turns out to be merged by mistake, or contain a regression, it may be reverted.
+A revert is not a rejection of the contribution, but merely part of an effective development process.
+It makes sure that development keeps running smoothly, with minimal uncertainty, and less overhead.
+If maintainers have to worry too much about avoiding reverts, they would not be able to merge as much.
+By embracing reverts as a good part of the development process, everyone wins.
+
+However, taking a step back may be frustrating, so maintainers will be extra supportive on the next try.