doc: Rework docs to align with Rust and other libs (#93)

* Initial README edit

* Update info on PR titles and commit messages

* Remove note about (IGNORE) in PRs

* Move generated API docs to api-docs dir, split README into 3 new files in docs dir

* Align with latest changes

* Heading reorg

* Make gh/doc links consistent

* trying to fix links

* Trying to fix README links

* Add link to supported formats

* Change title for consistencty

* Modify GH only blurb

* Review feedback from @tmathern

CONTRIBUTING.md

@@ -4,27 +4,27 @@ We welcome contributions to this project!
 
 Before you start, we ask that you understand the following guidelines.
 
-## Code of Conduct
+## Code of conduct
 
 This project adheres to the Adobe [code of conduct](CODE_OF_CONDUCT.md). By participating,
 you are expected to uphold this code. Please report unacceptable behavior to
 [[email protected]](mailto:[email protected]).
 
-## Have a Question?
+## Have a question?
 
 Start by filing an issue. The existing committers on this project work to reach
 consensus around project direction and issue solutions within issue threads
 (when appropriate).
 
-## Contributor License Agreement
+## Contributor license agreement
 
 All third-party contributions to this project must be accompanied by a signed contributor
 license agreement. This gives Adobe permission to redistribute your contributions
 as part of the project. [Sign our CLA](https://opensource.adobe.com/cla.html). You
 only need to submit an Adobe CLA one time, so if you have submitted one previously,
 you are good to go!
 
-## Code Reviews
+## Code reviews
 
 All submissions should come in the form of pull requests and need to be reviewed
 by project committers. Read [GitHub's pull request documentation](https://help.github.com/articles/about-pull-requests/)
@@ -36,7 +36,39 @@ These include such things as Rust code format, Clippy/lint checks, and unit test
 We encourage you to raise an issue in GitHub before starting work on a major addition to the crate.
 This will give us an opportunity to discuss API design and avoid duplicate efforts.
 
-## From Contributor to Committer
+### Pull request titles
+
+Titles of pull requests that target a long-lived branch such as _main_ or a release-specific branch should follow [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/#specification). This means that the first word of the pull request title should be one of the following:
+
+  * `build`
+  * `chore`
+  * `ci`
+  * `docs`
+  * `feat`
+  * `fix`
+  * `perf`
+  * `refactor`
+  * `revert`
+  * `style`
+  * `test`
+
+Optionally, but preferred, a scope can be added in parentheses after the type. The scope should be the name of the module or component that the commit affects. For example, `feat(api): Introduce a new API to validate 1.0 claims`.
+
+If more detail is warranted, add a blank line and then continue with sentences (these sentences should be punctuated as such) and paragraphs as needed to provide that detail. There is no need to word-wrap this message.
+
+For example:
+
+```text
+feat(api): Introduce a new API to validate 1.0 claims
+
+Repurpose existing v2 API for 0.8 compatibility (read: no validation) mode.
+```
+
+The conventional commit message requirement does not apply to individual commits within a pull request, provided that those commits will be squashed when the PR is merged and the resulting squash commit does follow the conventional commit requirement. This may require the person merging the PR to verify the commit message syntax when performing the squash merge.  
+
+TIP: For single-commit PRs, ensure the commit message conforms to the conventional commit requirement, since by default that will also be the title of the PR.
+
+## From contributor to committer
 
 We love contributions from our community! If you'd like to go a step beyond contributor
 and become a committer with full write access and a say in the project, you must
@@ -45,7 +77,7 @@ process that must reach lazy consensus (silence is approval) before invitations
 are issued. If you feel you are qualified and want to get more deeply involved,
 feel free to reach out to existing committers to have a conversation about that.
 
-## Security Issues
+## Security issues
 
 Do not create a public GitHub issue for any suspected security vulnerabilities. Instead, please file an issue through [Adobe's HackerOne page](https://hackerone.com/adobe?type=team). 
 For more information on reporting security issues, see [SECURITY.md](SECURITY.md).