pgsql: Rework installation instructions again

Rework installation instructions again

Dept. of second thoughts: the new verbiage added in commit aaec237b1a2f
is targeted at the wrong audience.  Remove the bits about git and talk
about how to get tarballs only.  People looking for the git repo can
look in the appendix.  That'll need to be expanded, but this commit
doesn't do that.

In passing, fix a couple of typos that snuck in with the previous
commit.

Discussion: https://postgr.es/m/713760.1626891263@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/73c5d2bfee2907b7f158485a531f00ba61b054f7

Modified Files
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doc/src/sgml/installation.sgml | 29 +++++++++--------------------
1 file changed, 9 insertions(+), 20 deletions(-)