NO_CHANGELOG=internal
NO_DOC=internal
NO_TEST=internal

GitHub suggests to look on the document, when it is present. The banner
is shown for visitors of issues and pull requests pages. We can use this
feature to highlight typical stones on the contributor's road.

My goal is to leave suggestions for typical points of a confusion in a
short document (don't bury into details) and leave links for further
reading.

I hope we should move most of content of the 'open a pull request'
section to a wiki page and possibly merge it with [1] and [2]. It
requires more work and I would consider it as a future step toward good
developer guidelines.

[1]: https://github.com/tarantool/tarantool/wiki/Code-review-procedure
[2]: https://www.tarantool.io/en/doc/latest/dev_guide/developer_guidelines

NO_CHANGELOG=not a user visible change
NO_DOC=nothing requires an update on the website
NO_TEST=it is purely documentation change

Well, the diff is not nice, so I'll summarize the changes for humans.

General changes:

* Moved patch submission and so on to the end of the document.
* Reformatted to usual 80 columns, inlined and actualized links.
* Reformatted lists: start from a capital letter, end with a period.
* Minor wording tweaks to make it sound better.

Badges:

* Travis CI and GitLab CI -> GitHub Actions.
* Removed Slack and Gitter.
* Google Groups -> GitHub Discussions.
* Added Stack Overflow.

Define Tarantool as in-memory computing platform as on the website.

Added license information.

Appserver:

* Rewrote the sentence regarding compatibility with Lua 5.1 and
  highlighted key LuaJIT features instead. There are doubts within the
  team about declaring 100% compatibility with Lua 5.1, let's go to the
  safe ground so.
* Splitted the list in a more granular way.
* Added links to mysql and pg connectors.
* Added queue, vshard, cartridge with links.
* Added a link to the modules page.

Database:

* Replaced MsgPack with MessagePack. It is called so on the official
  website.
* Replaced 'optional persistence' with 'complete WAL-based persistence'
  in the memtx description to make it less confusing. There are users,
  which believe that memtx does not offer persistency.
* Moved SQL down.
* Added JSON path indexes.
* Added synchronous quorum-based replication.
* Added RAFT-based automatic leader election.
* Added a link to the connectors list.

Platforms:

* Dropped OpenBSD from the list. It is not verified in CI, so I feel it
  like 'we have no commitments here'.
* Replaced x86 with more explicit x86_64.
* Moved Mac OS before FreeBSD (we have more users here).
* Added aarch64/M1.

Tarantool is ideal for...

* Kept intact, just reformatted to 80 columns.

Download:

* Added 'or using Docker' in addition to 'as a binary package to your
  OS'.

Added a link to the 'Awesome Tarantool' list.

Report bugs / send feedback or patch:

* Replaced Google Groups with GitHub Discussions.
* Added Stack Overflow.
* Highlighted that we accept pull requests and linked 'How to get
  involved' guide.

Fixes #6579

Part of #4967

Aside of that fixed other badges URLs to point to master branch.

(cherry picked from commit 7965aaaa88e6eaf266d2e51776598ac05a4a6cde)

2.0 was renamed to 2.1.

+ Travis CI
+ Telegram
+ Slack
+ Google Groups

* doc: moved to http://github.com/tarantool/doc

* Use our own fork
* Fix typo in CMakeLists.txt
* Update build dependecies in README.md
* Fix a potential bug with buffer size calculation in ss_lz4filter_next()

see https://cmake.org/cmake/help/v3.4/variable/CMAKE_BUILD_TYPE.html

Update mans, docs.

A lot of resources still need to be updated (build scripts, etc).