doc: Add documentation for using your own swc version

[vpanta]

additionally, updates mirror_releases.sh so it can be run to retrieve a single specific version and its hashes.

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Type of change

• Documentation (updates to documentation or READMEs)

For changes visible to end-users

• Relevant documentation has been updated

Test plan

• Manual testing; please provide instructions so we can reproduce:
  • Executed mirror_releases.sh both with a version to search for and without, to ensure it still worked in both cases.

[alexeagle]

Thanks!