docs improvements

## p1

- [x] https://github.com/iterative/cml/issues/514
- [ ] update landing page https://cml.dev
+ [ ] clear message around 2 major use cases: `runner` & `report`
* [ ] report: basic (no DVC) & advanced (lots of integrations, e.g. https://github.com/iterative/dvc/issues/6823)
* [ ] runner: cloud (AWS/Azure/GCP/K8s) & on-prem
+ [ ] diagrams, flowcharts, ... vis. @0x2b3bfa0's stacked crates
+ [x] depends on [cml#762](https://github.com/iterative/cml/issues/762)
- [ ] move/expand README content in cml.dev ([#718](https://github.com/iterative/cml/issues/718))
+ [ ] strip down & update README & redirect to website https://github.com/iterative/cml/issues/718
- [x] update Get Started #107
- [ ] put `cml ci` in Get Started/Usage
- [ ] put `cml pr` in Get Started/Usage
- [ ] dogfood https://github.com/iterative/cml/issues/1109
- [x] FAQs
+ [x] GH: fix PR branch checkouts #252
+ [x] #253
+ [x] #254
- [x] sync missing command ref options #256
- [ ] Clearly separate 3 different use cases that are subsequently merged into 1 e2e example
+ [ ] CI/Git helpers (`ci`, `pr`)
+ [ ] Reporting (`comment`, `tensorboard-dev`)
* [x] also depends on [cml#762](https://github.com/iterative/cml/issues/762)
+ [ ] BYOC: bring-your-own-cloud/compute (`runner`)

## p2

- [ ] update formatting (`cli` fenced blocks, ``intions) #256
- [ ] resurrect, update & move old wiki tutorials (TensorFlow MNIST [GH](https://github.com/iterative/cml/wiki/Tensorflow-Mnist-for-Github-Actions) & [GL](https://github.com/iterative/cml/wiki/Tensorflow-Mnist-for-Gitlab-CI)) to cml.dev
- [ ] check for old gems buried in https://github.com/iterative/cml/tree/dvc-cml
- [ ] links/references to awesome complementary GH/GL/BB features
+ [ ] cron schedule

Originally from https://github.com/iterative/cml/issues/514 & @daavoo feedback