Update docs site for version 2.21.0a0

[WorkerPants]

Docs from https://github.com/pantsbuild/pants/releases/tag/release_2.21.0a0

[huonw]

Currently failing with errors like the following, #195 seems to fix it (validated in #196)

> docusaurus build

[INFO] [en] Creating an optimized production build...
[info] [webpackbar] Compiling Client
[info] [webpackbar] Compiling Server
[success] [webpackbar] Client: Compiled successfully in 7.23m
[success] [webpackbar] Server: Compiled successfully in 22.90m

<--- Last few GCs --->

[2479:0x6662c50]  1497861 ms: Scavenge 6016.3 (6164.1) -> 6014.2 (6171.8) MB, 15.6 / 0.0 ms  (average mu = 0.224, current mu = 0.044) allocation failure; 
[2479:0x6662c50]  1497930 ms: Scavenge 6024.0 (6171.8) -> 6021.7 (6177.1) MB, 14.8 / 0.0 ms  (average mu = 0.224, current mu = 0.044) allocation failure; 
[2479:0x6662c50]  1500045 ms: Mark-sweep 6029.2 (6177.1) -> 5951.1 (6179.8) MB, 2073.8 / 0.0 ms  (average mu = 0.262, current mu = 0.299) allocation failure; scavenge might not succeed


<--- JS stacktrace --->

FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory
 1: 0xb9a330 node::Abort() [node]
 2: 0xaa07ee  [node]
 3: 0xd71ed0 v8::Utils::ReportOOMFailure(v8::internal::Isolate*, char const*, bool) [node]
 4: 0xd72277 v8::internal::V8::FatalProcessOutOfMemory(v8::internal::Isolate*, char const*, bool) [node]
 5: 0xf4f635  [node]
 6: 0xf50538 v8::internal::Heap::RecomputeLimits(v8::internal::GarbageCollector) [node]
 7: 0xf60a33  [node]
 8: 0xf618a8 v8::internal::Heap::CollectGarbage(v8::internal::AllocationSpace, v8::internal::GarbageCollectionReason, v8::GCCallbackFlags) [node]
 9: 0xf3c1fe v8::internal::HeapAllocator::AllocateRawWithLightRetrySlowPath(int, v8::internal::AllocationType, v8::internal::AllocationOrigin, v8::internal::AllocationAlignment) [node]
10: 0xf3d5c7 v8::internal::HeapAllocator::AllocateRawWithRetryOrFailSlowPath(int, v8::internal::AllocationType, v8::internal::AllocationOrigin, v8::internal::AllocationAlignment) [node]
11: 0xf1e7ca v8::internal::Factory::NewFillerObject(int, v8::internal::AllocationAlignment, v8::internal::AllocationType, v8::internal::AllocationOrigin) [node]
12: 0x12e3a1f v8::internal::Runtime_AllocateInYoungGeneration(int, unsigned long*, v8::internal::Isolate*) [node]
13: 0x1710739  [node]
Aborted (core dumped)