Extension of Solr documentation

[vkush]

Proposal for some updates and extensions for Solr documentation:

1. Reference Extended Solr guide already in the main README file, just for the case, when user wants to install Solr separately. The part where Solr config from webarchive-discovery is used to create Solr Cloud is very important.
2. Since Solr-9 it is not possible to store index files outside of Solr home folder (e.g. for the case, when external file storage is used), some additional setting of solr.allowPaths is needed for that, see here. This note should be mentioned in Extended Solr guide.
3. Usage of "--collection_id" in custom configuration of the indexer in the main README:
   

   solrwayback/README.md

   Line 228 in ba4d691

   THREADS=4 INDEXER_CUSTOM="--collection_id collection1 --collection corona2021" ./warc-indexer.sh warcs1/*

   
   Currently the spelling of "collection_id" was changed to "collection-id", see issue: "collection-id" vs "collection_id" in WARCIndexerCommandOptions.java ukwa/webarchive-discovery#315 . So, maybe with new release it will work again with "collection_id".
   Also for custom configuration the parameter "--institution institution1" also could be added as an example to the README file, to have all 3 possibilities. I think, we have only 3 options here: https://github.com/ukwa/webarchive-discovery/blob/13595bead029fd44f133ec6c18f689edde202e53/warc-indexer/src/main/java/uk/bl/wa/solr/SolrFields.java#L151-L154

[VictorHarbo]

Hi @vkush, thanks for your inputs! You are more than welcome to open documentation pull request for all three of your suggestions! For 1. and 2. I agree that these things should be mentioned exactly where you've described them. For 3. a note would indeed be helpful.