doc: add a script to ease experimenting with crud

[Totktonada]

Basic usage:

$ ./doc/playground.lua
tarantool> crud.select('developers', nil, {first = 6})
tarantool> crud.<something else>(<...>)

What the script is doing, step by step:

• Configure the database to don't leave WAL and snapshot files.
• Configure and bootstrap vshard storage and router on the single
  instance.
• Create a space and fill it with data.
• Start a console.

[Totktonada]

Created for myself to experiment around crud, but decided to share for comments. If it'll look useful for somebody, I'll merge it. If not, I'll close the pull request.

[oleg-jukovec]

Thank you for the pull request! It seems to me that this is a good idea. Now from README.md it is not clear how to manually test the crud. For example, I didn't understand how to run it manyally and tested it only with the luatest.

Manual testing will be very easy with this script.

I haven't looked at the code very carefully because it is a proposal. So I made my opinion and I noted only the obvious things:

1. Need a section in README.md as entry point.
2. Need autotests.

[Totktonada]

Still in the draft state, but updated a bit.

Changes:

• Removed help() / prev(). It does not feel very convenient for me. I would rather show a list of sections / examples to read, enumerate them and let a user to choose what to look. Commands would look like help list, help show 1. I'll not dig into this direction within this pull request. Let's look into docs in a separate window. But I opened PR Tiny documentation enhancements #291 to ease navigation over the documentation. BTW, we can expand the idea with help list / help show to make the examples testable.
• Added customers space. Its format and content more or less corresponds to what we can see in README.md. However there is no consistency in the readme itself: different examples assume different data (and even different bucket_count value). I decided to don't polish the readme in this pull request.
• Changed bucket_id values in the developers space to bucket_id_strcrc32() values calculated using primary keys. I would update them in doc/select.md and doc/pairs.md later. See Added new examples of select and pairs queries. #109 (comment).
• Added bucket_id indexes. It does not change anything for playground, but a real sharded space should have this index to allow vshard rebalancer doing its work.
• Created a PoC patch to fix annoying prompt/log mixing: console: don't mix stdout/stderr with readline prompt tarantool#7170.

TODO list:

• Mention in README.md.
• Add several simple tests: run playground, do some crud requests, check output.

[Totktonada]

Changes:

• Added 'Quickstart' section into README.md.
• Added a test.

It is ready for review now, but I need to choose between two variants of the test.

[oleg-jukovec]

Thanks for the update, I made a few notes, but they are not critical and can be easily corrected. So LGTM after resolving the notes.

[DifferentialOrange]

LGTM after resolving remaining comments.