Update README to match with unification changes

[JurajSadel]

Thank you!

Thank you for your contribution.
Please make sure that your submission includes the following:

Must

• The code compiles without errors or warnings.
• All examples work.
• cargo fmt was run.
• Your changes were added to the CHANGELOG.md in the proper section.
• You updated existing examples or added examples (if applicable).
• Added examples are checked in CI

Nice to have

• You add a description of your work to this PR.
• You added proper docs for your newly added features and code.

Removes the references to chip-specific HALs and briefly describes crates inside esp-hal. Also, for now, removes esp-hal documentation links.

closes #1209

[MabezDev]

A typo to fix, but LGTM. I'll let others chime in with regards to the content before merging.

[bjoernQ]

LGTM

[jessebraham]

Thanks!