doc: add documentation and rollups example to roadmap's vision

[Bidon15]

Notes on why imho 3 points should be included in the vision section:

Documentation is king for wider adoption

Currently, our documentation is very todo-listish and users(me included back in time) faced harshed realities of implementation for their projects' needs. Async hand-holding is needed as I've heard multiple times that nailing down the concept of testground is not straight-forward and is causing issues like:

• Docs: Role assignment/usage in test-case example #1336 - compositions are hard to nail down. Especially when a user opens the runenv APIs and discovers that there are multiple ways to treat the test-run instance's perspective
• Publishing from 1 instance out of N(rest subscribes only) hangs further test logic #1378 - classic Dunning-Kruger effect that should be populated somewhere in docs that blocking calls should be treated with extra-caution when using sync-service APIs
• empty results for executed test-plan #1275 - what is results exactly? Somebody can think like me that it's the pass/failure which is not the case

Large-scale networks(example for a rollup)

Going forward with the L1-L2-L(N) narrative, it will be beneficial if we think forward what users might need from testground to test their rollups, or even a complete e2e system test, like:
A Button call-> Triggers a smart contract -> state transition in the rollup -> dumps data to a DA Layer. And then we check the new data at the UI level. Now, imagine how many variations we can have for a single test-case (network wise as well as VM, DA, instances count, etc.)

Selenium/Cypress

Both are golden standards for web-browser automation and I do believe that testground should be a one-stop shop for blockchains' / p2p network testing/experimentation

Ref: #1491

[Bidon15]

I haven't added this into the PR but leaving another point here:
It's community plugins for testground being featured in the repo

[laurentsenta]

(rebased on latest master)