docs: refresh contributing & building guides, surface CLA signing flow (#15503)

.github/CLA.md

@@ -1,4 +1,4 @@
-<!-- To indicate your agreement, simply edit this file and submit a pull request. -->
+<!-- Do NOT edit this file to sign. To sign the CLA, visit https://cla-assistant.io/toeverything/AFFiNE and sign in with GitHub. -->
 
 # AFFiNE Contributor License Agreement
 
@@ -35,4 +35,6 @@ You accept and agree to the following terms and conditions for your past, presen
 
 ## How To Sign
 
-Visit https://cla-assistant.io/toeverything/AFFiNE and sign it.
+Visit **<https://cla-assistant.io/toeverything/AFFiNE>**, sign in with your GitHub account, and agree.
+
+Once signed, the `license/cla` check on your pull requests passes automatically. If you signed after opening a PR, click the **recheck** link in the CLA bot's comment on the PR (or push a new commit) to refresh the check. Note that every committer on a PR must sign, and each commit's author email must be linked to a GitHub account.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+<!--
+Thank you for contributing to AFFiNE!
+
+The PR title must follow Conventional Commits (enforced by CI):
+  type(scope): description      e.g.  fix(editor): keep selection after paste
+Types: feat fix docs style refactor perf test build ci chore revert
+-->
+
+## Description
+
+<!-- What does this PR do? Link related issues, e.g. "Closes #1234". Screenshots or recordings are welcome for UI changes. -->
+
+## Checklist
+
+- [ ] I have signed the [AFFiNE Contributor License Agreement](https://cla-assistant.io/toeverything/AFFiNE) — required before merge; the `license/cla` check must be green ([how it works](https://github.com/toeverything/AFFiNE/blob/canary/docs/BUILDING.md#sign-the-cla-first))
+- [ ] The PR targets the `canary` branch and its title follows [Conventional Commits](https://www.conventionalcommits.org/)
+- [ ] Tests are added or updated where it makes sense
+- [ ] `yarn lint` and `yarn typecheck` pass locally