Improve docs on SCRATCHDIR

[janxkoci]

Some formulations in the docs describing the use of SCRATCHDIR are misleading, e.g.:

metacentrum-user-docs/content/docs/computing/run-basic-job.mdx

Lines 109 to 113 in 5981b58

	The last two lines can be piped together.
	```bash
	cp h2o.out $DATADIR/ || export CLEAN_SCRATCH=false
	```

It is misleading / incomplete, because the suggested code does not call clean_scratch, so the scratch is not actually cleaned. I believe the line should instead read:

cp h2o.out $DATADIR/ || export CLEAN_SCRATCH=false; clean_scratch

A simpler alternative suggested by support staff by email could be included in the docs too:

cp h2o.out $DATADIR/ && clean_scratch

This cleans scratch only if the copy command succeeds.

Similar examples could be also added to the dedicated page on SCRATCHDIR, which currently looks very incomplete (to a user searching information about using scratch): https://docs.metacentrum.cz/en/docs/computing/infrastructure/scratch-storages

[janxkoci]

Another problem shows up when using scratch with job arrays, where the variable $SCRATCHDIR contains special characters []. This means the variable now must be quoted, i.e. "$SCRATCHDIR", or else using it with e.g. cp will always fail!

This should be properly documented, as it has bitten me and a few colleagues already.