Add comprehensive fee estimation documentation and examples for all supported backends

[VolodymyrBg]

Description

This PR adds comprehensive documentation and real-world code examples for transaction fee estimation using all major blockchain backends supported by BDK: Electrum, Esplora (blocking and async), and bitcoind RPC.
A new file, docs/fee_estimation.md, has been created with clear, minimal, and production-relevant examples for each backend.
Additionally, a direct link to this documentation has been added to the "About" section of the main README.md for better discoverability.
This PR completes the outstanding documentation task from bdk #289 as described in bdk_wallet#140.

Notes to the reviewers

• The new documentation is based on actual API usage and project discussions, ensuring accuracy and relevance.
• The link in the README makes it easy for developers to find fee estimation guidance without cluttering the main documentation.
• No code logic was changed; this is a documentation-only PR.

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[thunderbiscuit]

I think this type of documentation belongs in the bookofbdk.com project. Would you mind opening up a PR with this content so we can discuss it there instead?

https://github.com/bitcoindevkit/book-of-bdk