Port docs from spl

[joncinque]

Problem

It's time to move over the SPL docs, but it hasn't been done.

Summary of changes

Following the PR for the feature proposal program at solana-program/feature-proposal#1, but for the docs. I ran:

# add the remote for the source repo
git remote add spl https://github.com/solana-labs/solana-program-library.git
# fetch the master from spl under a different local branch
git fetch spl master:fromspl
git switch fromspl
# be safe, remove the remote
git remote rm spl
# filter the stake related files
python3 ../../git-filter-repo \
  --path docs \
  --force
# re-add this repo
git remote add origin [email protected]:joncinque/solana-program.github.io.git
git fetch origin main
git switch main
git switch fromspl
git rebase main