doc: create a github actions job to check docs generated for warnings

[dj8yfo]

it should run RUSTDOCFLAGS="-D warnings" cargo doc -p near-sdk --features unstable,legacy,unit-testing,__macro-docs
and some broken links should be fixed.

near-sdk-macros doc generation should be checked too

[dj8yfo]

it was noticed in scope of #1259, if new intra-doc links are added , they will be automatically detected for being broken (if they are),
via RUSTDOCFLAGS="-D warnings"

error: unresolved link to `ear_vm_runner::logic::VMLogic::promise_batch_action_function_call_weight`
   --> near-sdk/src/environment/env.rs:998:50
    |
998 | ...t [ear_vm_runner::logic::VMLogic::promise_batch_action_function_call_weight]
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `ear_vm_runner` in scope
    |
    = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`