Add initial cluster docs

[bjosv]

This is the cluster overview-docs similar to the existing standalone.md.
Most info taken from the old readme, but reusing the structure from the standalone docs.

Additionally:

• Update wordlist for spellcheck
• Enable manual CI-runs of spellcheck.
• Correcting link in standalone.md.
• Move gencommands.py to scripts/ (so that the link in docs works..)

[zuiderkwast]

Great!

I didn't proof-read all of it, but assuming it's mainly hiredis => libvalkey. Or is lots of it rewritten?

Can you delete the corresponding docs in the old README, so we know what's left there to handle later?

[bjosv]

Can you delete the corresponding docs in the old README, so we know what's left there to handle later?

I will attempt to clean-up/remove the libvalkey and libvalkeycluster directories next, I'll see if there is anything left to add.