docs: Update links to jinja docs

[bengerman13]

jinja now lives on palletsproject.com, not pocoo.org.

One thing of note is that these links are coded for English, which is a little opinionated. On the other hand, this site is in English and Pallets doesn't seem to have other languages currently.

[dairiki]

One thing of note is that these links are coded for English, which is a little opinionated.

Heuristically, it appears that https://jinja.palletsprojects.com/templates/ redirects to https://jinja.palletsprojects.com/en/stable/templates/, so the hard-coded en could be avoided.

Almost any other path will result in an infinite redirect hole, however, so this may not be the safest behavior to rely upon.

$ curl -Lv https://jinja.palletsprojects.com/xxx/ 2>&1 | grep location:
< location: https://jinja.palletsprojects.com/en/3.1.x/xxx/
< location: https://jinja.palletsprojects.com/en/stable/xxx/
< location: https://jinja.palletsprojects.com/en/3.1.x/en/stable/xxx/
< location: https://jinja.palletsprojects.com/en/stable/en/stable/xxx/
< location: https://jinja.palletsprojects.com/en/3.1.x/en/stable/en/stable/xxx/
< location: https://jinja.palletsprojects.com/en/stable/en/stable/en/stable/xxx/
< location: https://jinja.palletsprojects.com/en/3.1.x/en/stable/en/stable/en/stable/xxx/
< location: https://jinja.palletsprojects.com/en/stable/en/stable/en/stable/en/stable/xxx/
< location: https://jinja.palletsprojects.com/en/3.1.x/en/stable/en/stable/en/stable/en/stable/xxx/
[...]

On the other hand, this site is in English and Pallets doesn't seem to have other languages currently.

Given the above, I agree that the hard-coded en is not particularly an issue.

[dairiki]

Further poking reveals that jinja.palletsprojects.com points directly to readthedocs.io.

Read the Docs supports auto-redirects for specific pages from /page/... to the default language and version of the hosted documentation.
Experimentally, https://jinja.palletsprojects.com/page/templates/ does, indeed, redirect to the right place.

Given that this is documented behavior, it doesn't seem unreasonable to use /page/templates/ in place of /en/stable/templates/.

$ dig +noall +answer jinja.palletsprojects.com
jinja.palletsprojects.com. 17	IN	CNAME	readthedocs.io.
readthedocs.io.		17	IN	A	104.16.254.120
readthedocs.io.		17	IN	A	104.16.253.120

$ curl -Lv https://jinja.palletsprojects.com/page/templates/ 2>&1 | grep location:
< location: https://jinja.palletsprojects.com/en/stable/templates/