Attempt to migrate external repo docs to new format

[Simran-B]

This PR adds scripts/fetchRefs.sh, a reworked version of the script we used to sync external repository docs into the main repo, but it can now be run in the docs repo (e.g. using Git Bash):

cd scripts ./fetchRefs.sh git 

It also modifies the migration script to replace https://www.arangodb.com/docs/stable/... links by relative links. There are some flaws however, e.g. the following incorrect conversion (arangojs docs/Drivers/JS/GettingStarted/README.md):

• Source: [_errorNum_ error code](https://www.arangodb.com/docs/stable/appendix-error-codes.html)
• Wrong: [_errorNum_ error code](../appendix/error-codes.html)
• Correct: [_errorNum_ error code](../appendix-error-codes.html)

The unstaged changes need to be compared manually, and only real and correct updates need to be staged. This will hopefully change in the future.

Related: #143

• Java driver Update Docs arangodb-java-driver#318
• ArangoJS Doc updates for syncing arangojs#631
• PHP driver: Docs: Update links, formatting arangodb-php#270
• Go driver:
  formatting update arangodb/go-driver@64c417a
  logo link arangodb/go-driver@4fcd60d
  line break arangodb/go-driver@9853509
• Spring data: Docs: Update links spring-data#161
  compat table was updated
• Spark connector: arangodb/arangodb-spark-connector@2fe7f35 / arangodb/arangodb-spark-connector@6d0c3ab
• Starter Docs: Some adjustments for syncing arangodb-helper/arangodb#244
• Arangosync: https://github.com/arangodb/arangosync/pull/459
• K8s operator Docs: Update links docs.arangodb.com -> www.arangodb.com/docs kube-arangodb#488
  rClone -> Rclone, here link, hidden cross-links (relative)

Notes:

• local .html links won't work on GitHub...
  • docs.arangodb.com link logic needs to be applied to www.arangodb.com/docs as well, to turn absolute into relative links
  • convert relative to absolute links in external repos if they point to main docs
• descriptions: multiple lines need to be joined with a space