LU-16751 docs: consolidate and cleanup READMEs

LU-16751 docs: consolidate and cleanup READMEs

A number of the in-tree READMEs are very outdated. Consolidating
these into the top-level README will make it more likely that the
infomation is read (since this file gets rendered by many git hosts)
and more likely to be keep up-to-date.

The information in the consolidated README has been updated.

Some docs which don't seem relevant anymore are simply deleted.

The llverfs.txt doc has been converted to a proper man page. The
descriptive comment in llverfs.c has been redirected towards the
man page instead, to reduce the risk of these becoming out-of-sync.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I57a6f13056913551d96363ffdbce76beed5c9486
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50703
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>