* Document _URL/_SAVE_AS path matching req. Fix #2158

This commit addresses the fact that some URLs (primarily {attach}) are based
on the *_SAVE_AS path instead of the *_URL path. It updates the documentation
to point out that the paths of corresponding _URL and _SAVE_AS settings should
match, and that the OUTPUT directory should correspond to the web server's
document root, in order to avoid producing these incorrect URLs.

* Clarify {attach} and *_URL / *_SAVE_AS caveat

Co-authored-by: Justin Mayer <entroP@gmail.com>

Pelican has a setting for custom Jinja filters but not custom tests or
custom environment namespace globals. This adds both of the latter.

This also updates the Tumblr API to use HTTPS as documented in the
current Tumblr API docs.

doc: Add cross links between theme settings and pelican-themes

Fix a typo in the pagination settings docs

Add custom summary end marker