docs: broken/invalid links

[LesnyRumcajs]

Running cargo doc shows some issues with the existing documentation.

Sample:

warning: `fvm_sdk` (lib doc) generated 3 warnings
warning: unresolved link to `fvm_shared::ExitCode::SYS_ASSERTION_FAILED`
  --> fvm/src/syscalls/error.rs:25:34
   |
25 |     /// [`SYS_ASSERTION_FAILED`][fvm_shared::ExitCode::SYS_ASSERTION_FAILED] exit code on-chain.
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `ExitCode` in module `fvm_shared`
   |
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

warning: unresolved link to `GasTimer::finish_with`
  --> fvm/src/gas/timer.rs:68:36
   |
68 |     /// Use the return value with [GasTimer::finish_with] to override the internal
   |                                    ^^^^^^^^^^^^^^^^^^^^^ the struct `GasTimer` has no field or associated item named `finish_with`

warning: this URL is not a hyperlink
 --> fvm/src/account_actor.rs:9:26
  |
9 | //! eliminated. Refer to https://github.com/filecoin-project/fvm/issues/229 for
  |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: bare URLs are not automatically turned into clickable links
  = note: `#[warn(rustdoc::bare_urls)]` on by default
help: use an automatic link instead
  |
9 | //! eliminated. Refer to <https://github.com/filecoin-project/fvm/issues/229> for
  |                          +                                                  +

It would be great to resolve those issues and ensure that they don't happen again as part of CI.