Add warning to jaeger docs about future deprecation:

[cijothomas]

Towards #995.
This does not deprecate anything. Instead a warning about a future deprecation is added to the docs, so users can start switch to OTLP one sooner.

Note:
rust-lang/rust#106561 is not yet merged, so I could not find any better way to put a "Warning" thing into the doc. Open to any suggestions!

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.1 ⚠️

Comparison is base (62450b9) 69.9% compared to head (7ac0b25) 69.9%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #996     +/-   ##
=======================================
- Coverage   69.9%   69.9%   -0.1%     
=======================================
  Files        116     116             
  Lines       8922    8922             
=======================================
- Hits        6245    6239      -6     
- Misses      2677    2683      +6     

Impacted Files	Coverage Δ
opentelemetry-jaeger/src/lib.rs	93.3% <ø> (ø)

... and 1 file with indirect coverage changes

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[hdost]

We've been talking about creating a contrib potentially even if after we decide to remove we could always move it to contrib.