Fix broken intra-doc links + add CI stage to test example docs

[cmichi]

You can verify for yourself by executing

• cargo doc --workspace --document-private-items --verbose --no-deps in ink/ or

• cargo doc --manifest-path=Cargo.toml --document-private-items --verbose --no-deps in an example folder.

During the issue I discovered a bug in rustdoc.

[codecov-io]

Codecov Report

Merging #606 (d65b466) into master (5115146) will decrease coverage by 15.54%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #606       +/-   ##
===========================================
- Coverage   83.75%   68.21%   -15.55%     
===========================================
  Files         156      156               
  Lines        6969     6969               
===========================================
- Hits         5837     4754     -1083     
- Misses       1132     2215     +1083     

Impacted Files	Coverage Δ
crates/env/src/backend.rs	0.00% <ø> (ø)
crates/env/src/engine/off_chain/db/block.rs	100.00% <ø> (ø)
crates/lang/codegen/src/generator/cross_calling.rs	0.00% <ø> (-94.00%)	⬇️
crates/storage/src/collections/bitstash/mod.rs	97.91% <ø> (ø)
crates/storage/src/collections/smallvec/mod.rs	96.47% <ø> (ø)
crates/storage/src/collections/stash/mod.rs	96.81% <ø> (ø)
crates/storage/src/collections/vec/mod.rs	96.77% <ø> (ø)
crates/storage/src/lazy/entry.rs	94.73% <ø> (ø)
crates/storage/src/lazy/lazy_hmap.rs	93.77% <ø> (ø)
crates/lang/codegen/src/traits.rs	0.00% <0.00%> (-100.00%)	⬇️
... and 36 more

---

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[cmichi]

On my machine the latest rustc nightly (from yesterday) results in this ICE when executing cargo doc --workspace --verbose --no-deps in ink/: rust-lang/rust#81395. There are no issues in our CI though, since it uses the nightly from 2021-01-07.

I would hold merging this PR until the ICE is fixed.

[jryans]

I would hold merging this PR until the ICE is fixed.

The 2021-02-01 nightly includes a fix for this.

[Robbepop]

sorry for being so pedantic but simply accept my suggestions and we are good to merge

[Robbepop]

LGTM