fix: cargo publish errors with reference to docs/ folder

[dj8yfo]

with how a published crate source code is layed out, it probably cannot reference resources at workspace level https://docs.rs/crate/borsh/1.5.3/source/ .

a similar error to release-plz error in run https://github.com/near/borsh-rs/actions/runs/12746835799/job/35523991694,
is encountered when running cargo publish --dry-run on this pr's base branch:

❯ : cargo publish --dry-run
    Updating crates.io index
   Packaging borsh v1.5.4 (/home/user/near/borsh-rs/borsh)
    Updating crates.io index
    Packaged 84 files, 321.8KiB (60.8KiB compressed)
   Verifying borsh v1.5.4 (/home/user/near/borsh-rs/borsh)
   Compiling cfg_aliases v0.2.1
   Compiling borsh v1.5.4 (/home/user/near/borsh-rs/target/packag
e/borsh-1.5.4)
error: couldn't read `src/../../docs/rustdoc_include/borsh_crate_top_level.md`: No such file or dir
ectory (os error 2)
 --> src/lib.rs:2:10
  |
2 | #![doc = include_str!("../../docs/rustdoc_include/borsh_crate_top_level.md")]
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the macro `include_str` (in Nightly builds, run with -Z macro-ba
cktrace for more info)
help: there is a file with the same name in a different directory
  |
2 | #![doc = include_str!("../../../../docs/rustdoc_include/borsh_crate_top_level.md")]
  |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

error: could not compile `borsh` (lib) due to 1 previous error
error: failed to verify package tarball

the error goes away in target branch of this pr, so hopefully this will allow to release 1.5.4 borsh

[dj8yfo]

@race-of-sloths include

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