[7/7][sl][github][gh stacks] update some documentation

[raydatray]

ctx

documentation updates for the new native stacked workflow

changes made

• extension help text for github.pr-workflow (overlap/single/stacked) and pr submit --restack
• website: github.md + sapling-stack.md updated to describe the stacked workflow and its tradeoffs

test plan

docs only

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Stack created with Sapling. Best reviewed with ReviewStack.

• -> [7/7][sl][github][gh stacks] update some documentation #1398
• [6/n][sl][github][gh stacks] wire up gh stacks to sl pull #1397
• [5/n][sl][github][gh stacks] wire up gh stacks to sl pr submit #1396
• [4/n][sl][github][gh stacks] update submit body for stacks #1395
• [3/n][sl][github][gh stacks] add stacked workflow for submit #1394
• [2/n][sl][github][gh stacks] add support for stack endpoint #1393
• [1/n][sl][github][gh stacks] support headers and list params #1392

[meta-codesync]

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