chore(docs): core/sdk lib.rs and IggyClient docs

[haubur]

Draft PR as discussed here: https://discord.com/channels/1144142576266530928/1144142825630474360/1507361396382240839

Would be nice to get some feedback on:

• language (tone towards the user)
• understandability (level of detail)
• guidance (easy to follow examples)

If the general direction is good and supported, happy to clean and merge.
Would suggest to continue with IggyConsumer and IggyProducer in a next PR.

Which issue does this PR address?

Closes #3306

Rationale

Adding User facing docs for the Rust SDK.

What changed?

• proposal for crate level docs in lib.rs
• docs for the IggyClient type

Local Execution

• prek run (specifically typo and rust fmt)

AI Usage

Everything is manually written or at least re-written in cases I had claude suggest a formulation.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.46%. Comparing base (01a64b2) to head (b2496d2).
⚠️ Report is 35 commits behind head on master.

Files with missing lines	Patch %	Lines
core/sdk/src/clients/client.rs	81.81%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3809      +/-   ##
============================================
+ Coverage     76.58%   81.46%   +4.87%     
- Complexity     1046     1299     +253     
============================================
  Files          1347     1201     -146     
  Lines        171018   156998   -14020     
  Branches     142372   126432   -15940     
============================================
- Hits         130967   127891    -3076     
+ Misses        36233    25253   -10980     
- Partials       3818     3854      +36     

Components	Coverage Δ
Rust Core	81.73% <81.81%> (+5.95%)	⬆️
Java SDK	66.06% <ø> (+2.39%)	⬆️
C# SDK	74.73% <ø> (+2.45%)	⬆️
Python SDK	89.98% <ø> (+1.28%)	⬆️
PHP SDK	84.26% <ø> (+1.28%)	⬆️
Node SDK	95.54% <ø> (-0.74%)	⬇️
Go SDK	68.53% <ø> (-0.65%)	⬇️

Files with missing lines	Coverage Δ
core/common/src/types/message/partitioning.rs	72.99% <ø> (-2.19%)	⬇️
core/sdk/src/clients/client.rs	88.42% <81.81%> (-0.81%)	⬇️

... and 652 files with indirect coverage changes

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[slbotbm]

Will read more later

[haubur]

/ready

[haubur]

/ready