roadmap: Update improvement plans for upgrading huge sites
roadmap: Update improvement plans for upgrading huge sites

Add offline reindex to the list of improvement areas, add links to the
hashtags tracking improvements, add a link to the Qualcomm upgrade mail
list thread, update current timings, update overall goal time, and note
that improvements are expected to continue through Q1 2022.

Change-Id: I0f25b86512d774721a29442e32adf87ce9557665

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