Update #1021 with a Makefile fix.

[ctb]

Adds .PHONY to the 'doc' target so as to force a docs build attempt.

This is a PR into #1021.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1053 into master will increase coverage by 9.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1053      +/-   ##
==========================================
+ Coverage   83.30%   92.42%   +9.12%     
==========================================
  Files          97       72      -25     
  Lines        8749     5454    -3295     
==========================================
- Hits         7288     5041    -2247     
+ Misses       1461      413    -1048     

Flag	Coverage Δ
#rusttests	?

Impacted Files	Coverage Δ
src/core/src/ffi/mod.rs
src/core/src/index/bigsi.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/wasm.rs
src/core/tests/test.rs
src/core/tests/minhash.rs
src/core/src/sketch/ukhs.rs
src/core/src/index/storage.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/from.rs
... and 15 more

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