Speed up Mill Docsite generation

[lihaoyi]

time ./mill -i docs.fastPages takes 1-2 minutes now on my laptop for incremental updates. This is unacceptable for something as visual a documentation. A large portion of this appears to be the graphviz diagram generation, but that is unavoidable as we want diagrams in our documentation. The core problem seems that our Antora setup doesn't seem to be caching and re-using any of the processed asciidoc files

We should figure out how to make Antora faster, or replace Antora entirely with something else that is faster.

[lihaoyi]

Also the asciidoctor-kroki uses a remote server to render the graphviz diagrams, which apart from being terribly slow is also unstable and returns 504s sometimes

[18:12:36.543] WARN (asciidoctor):
    file: /Users/lihaoyi/Github/mill/out/docs/devAntoraSources.dest/modules/ROOT/pages/Cross_Builds.adoc
    source: /Users/lihaoyi/Github/mill (branch: docs-overhaul <worktree> | start path: out/docs/devAntoraSources.dest)
    msg: {
      "err": {
        "package": "asciidoctor-kroki",
        "message": "Skipping graphviz block.",
        "stack": "Error: Skipping graphviz block.\n    at wrapError (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/asciidoctor-kroki.js:48:22)\n    at constructor.<anonymous> (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/asciidoctor-kroki.js:186:30)\n    at Object.apply (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/@antora/asciidoc-loader/node_modules/@asciidoctor/core/dist/node/asciidoctor.js:26192:21)\n    at constructor.$$instance_exec (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-opal-runtime/src/opal.js:3846:24)\n    at Opal.send (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-opal-runtime/src/opal.js:1671:19)\n    at Proxy.$$19 (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/@antora/asciidoc-loader/node_modules/@asciidoctor/core/dist/node/asciidoctor.js:19974:24)\n    at Opal.send (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-opal-runtime/src/opal.js:1671:19)\n    at constructor.$$process (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/@antora/asciidoc-loader/node_modules/@asciidoctor/core/dist/node/asciidoctor.js:19978:20)\n    at constructor.$$call (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-opal-runtime/src/opal.js:17898:26)\n    at Opal.send (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-opal-runtime/src/opal.js:1671:19)\nCaused by: Error: GET https://kroki.io/graphviz/svg/eNqN0bsOwiAUBuDdpzhxtwHUEVcXJx1NY0BOC5GIARprjO9urYmXiOIEnPx8h4sytRcHDXM4DwC82O-U8Xyx7BZ7pxDWQYsDculaOBoVNSeg0dQ6dpMQTxZ5ZaxFBbdh66zz_KhNxLIDQiPv-NY2IaLfsL4JgBUSLR9Wzq1ZQVk57Kt3TomgUfWFZ6AITVWZ9pa7pGCahGkOplmYJGGSg0kWnrzDUvjfJ34ECmnq1W97mrRZzmb_2OOkTXI2SdifjwWj2dcdn3_2mqZf0iyZfr_p5QoAaOhq - server returns 504 status code; response: \n    at httpRequest (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/http/http-client.js:45:9)\n    at Object.httpGet [as get] (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/http/http-client.js:53:10)\n    at Object.httpGet [as get] (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/http/node-http.js:5:65)\n    at KrokiClient.getImage (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/kroki-client.js:70:30)\n    at module.exports.save (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/fetch.js:47:78)\n    at createImageSrc (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/asciidoctor-kroki.js:67:38)\n    at processKroki (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/asciidoctor-kroki.js:163:25)\n    at constructor.<anonymous> (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-kroki/src/asciidoctor-kroki.js:184:16)\n    at Object.apply (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/@antora/asciidoc-loader/node_modules/@asciidoctor/core/dist/node/asciidoctor.js:26192:21)\n    at constructor.$$instance_exec (/Users/lihaoyi/Github/mill/out/docs/npmBase.dest/node_modules/asciidoctor-opal-runtime/src/opal.js:3846:24)"
      }
    }

I didn't manage to find any other graphviz diagram plugin for antora, but maybe i didnt look hard enough. https://docs.asciidoctor.org/diagram-extension/latest/ seems to be ruby-only and antora uses asciidoctor-js