docs: add testnets concepts & reference pages

[leohhhn]

Description

Closes: #1791 & #1822

This PR adds concept & reference pages for Gno testnets. It details out what each testnet is meant for, what its versioning strategy is, and how it handles state & tx history persistence.

I am missing some information -

• Since test4 will be versioned based on Gno releases, will this also include new code in the examples/ folder? How will this work? cc @zivkovicmilos
• Please comment below if you know the release dates of test1, test2, & test3 - and possibly the version (gh commit) of their binaries. found tags thanks to @r3v4s on the repo.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.74%. Comparing base (831bb6f) to head (d8f64f3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1867   +/-   ##
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  Coverage   47.74%   47.74%           
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  Files         393      393           
  Lines       61629    61629           
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  Hits        29424    29424           
  Misses      29734    29734           
  Partials     2471     2471           

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[zivkovicmilos]

Overal looks good 💯

Left some comments that should be addressed before merging

[zivkovicmilos]

Thank you for resolving the comments, please check what's leftover 🙏