Fix docs SIGILL by dropping ccache from the docs build

[zfergus]

Description

Removes ccache from the docs workflow, so ipctk is built from scratch on every docs run.

• Removed: the ccache install, the docs-cache restore step, and the stats steps in docs.yml.
• Also removed: the restore-keys: Linux-Python-cache fallback, which could restore an entry from a different ISA.

Impact:

• Build ipctk takes about 307 s instead of 40–64 s, so a docs run takes about 443 s instead of about 175 s. Docs only build on pushes to main and docs.
• No source or API changes, and no effect on the generated docs.

Why

• import ipctk crashes with SIGILL when Sphinx starts. conf.py imports the module at the top, so make html died after about 3 s (Build docs, exit code 132).
• Three runs restored the same docs-cache entry, which has been immutable since 10-07: the runs on 10-11 and 10-12 failed with SIGILL, and a third on 10-12 passed. The outcome depends on which VM runs the job, not on the commit.
• ccache hashes the literal -march=native, not what it expands to. -march=native comes from FindSIMD (SIMD_CXX_FLAGS) and Tight-Inclusion (its own target, no off switch). Turning off IPC_TOOLKIT_WITH_SIMD would drop the first but not the second.
• With no cache, no object built on one VM can be restored onto another.
• The same crash class is addressed for the other workflows in Fix CI ccache SIGILL by keying on the resolved ISA #269. They keep a cache, so they use an ISA-keyed one instead.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

docs.yml only runs on pushes to main and docs, and its last step deploys to gh-pages, so PR CI does not exercise it. The first run on main after merging is the real test.

• The workflow parses as YAML, every run block passes bash -n, and no step references ccache or actions/cache.
• Timings come from the existing runs: cold 443 s on 10-07 versus 168–177 s with the cache.

Test Configuration:

• OS and Version: macOS 26.6.2
• Compiler and Version: n/a (workflow-only change)

Checklist

• I have followed the project style guide
• My code follows the clang-format style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules