Document FastMCP server instructions

[MukundaKatta]

Adds a README section showing how to use FastMCP instructions to communicate workflow guidance and tool ordering/grouping context to clients through InitializeResult.\n\nCloses #1464.\n\nVerification:\n- Documentation-only change

[ryanwilliams90]

Hi @MukundaKatta — I started on #1464 today and posted a plan there before noticing this PR was already open. Apologies for the overlap.

Happy to defer if this lands. If it would be useful, I have a slightly broader take that I'm happy to either fold into this PR (with credit) or open separately if maintainers prefer:

• A runnable snippet file (examples/snippets/servers/instructions_example.py) wired through update_readme_snippets.py so the README block stays in sync the same way as the other server examples.
• A worked example covering the bundling / sequencing case the original issue calls out (search → fetch → confirm → update), plus explicit guidance on what belongs in instructions vs. tool docstrings — that's the design call I find people get wrong most often.

@maxisbey — you green-lit the docs work on #1464; happy to be guided on which direction is most useful. I can also just leave review comments here if you'd rather land this PR as-is.