docs: add a link to the list of all guides
docs: add a link to the list of all guides

Since git/git@f442f28a81 (git.txt: add list of guides, 2020-08-05), the
git(1) man page includes a list of all Git guides.

Add a link to this section of the man page at the bottom of the 'Guides'
section of the main documentation page, to improve discoverability.

mosoft521/gitscm-next,git/git-scm.com,Mokolea/git-scm.com,git/git-scm.com,git/git-scm.com,mosoft521/gitscm-next,Mokolea/git-scm.com,git/git-scm.com,Mokolea/git-scm.com,mosoft521/gitscm-next,Mokolea/git-scm.com,mosoft521/gitscm-next