docs: Update SWHID acronym to SoftWare Hash IDentifiers
Use updated SWHID acronym everywhere in the swh-model documentation.
Rename persistent-identifiers.rst to swhids.rst;
Use swhids reference instead of persistent-identifiers.
Depends on swh-docs!396 to setup redirection of previous URL for SWHIDs documentation page to the new one.
Edited by Antoine Lambert