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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.02%. Comparing base (01a64b2) to head (dd7440b).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3809      +/-   ##
============================================
- Coverage     76.58%   75.02%   -1.56%     
  Complexity     1046     1046              
============================================
  Files          1347     1347              
  Lines        171018   165645    -5373     
  Branches     142372   137074    -5298     
============================================
- Hits         130967   124283    -6684     
- Misses        36233    37241    +1008     
- Partials       3818     4121     +303     

Components	Coverage Δ
Rust Core	73.89% <ø> (-1.89%)	⬇️
Java SDK	63.67% <ø> (ø)
C# SDK	71.16% <ø> (-1.13%)	⬇️
Python SDK	88.70% <ø> (ø)
PHP SDK	82.97% <ø> (ø)
Node SDK	96.36% <ø> (+0.08%)	⬆️
Go SDK	69.18% <ø> (ø)

Files with missing lines	Coverage Δ
core/common/src/types/message/partitioning.rs	75.18% <ø> (ø)
core/sdk/src/clients/client.rs	89.22% <ø> (ø)

... and 152 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[slbotbm]

Will read more later

[haubur]

/ready