failures unrelated b/c this is pure docs

So, of course this PR has run away into some bigger content changes 🤦‍♀️:

• upgrades to sphinx 7.4.0 branch (this may require updates to sphinx gallery)
• added link to devel/index to landingpage contribute copy
• changes section title: "new contributors"->"contributing guide"
  • am slightly ambivalent about this change, but am thinking maybe it'll be more read if it's not cast as only for newbies b/c it's purpose is more so to provide information on what can be contributed, where/how, and what resources are available for contributors
• pulled some copy out of triage to anchor the new "issues" section.
• tightened up the "submit bug" and "new issue" buttons cause I thought the stuff I cut was either redundant or applied to both.
• inspired by new issues copy, unraveled the "new contributors should xyz" paragraph to instead add copy to each section on why someone should read it -> new contributors get guidance and experienced contributors can skim/skip to links.

Like I mentioned in this comment, I'm trying to avoid major restructure territory because I've got a rough plan (that yes, I figure will be up for discussion) for more clean up/polishing and this was supposed to be a "yay, new CSS style!" PR:

• waiting on Add branch tracking to development workflow instructions #28575, [Doc]: helping new contributors with choosing issues to work on  #28476, and Doc: added triage section to new contributor docs #28347 for integrating "how to PR" into "dev workflow"
• Doc: doc docs reorg #28578 for cleaning up the doc docs
• small rework on the triage docs. Maybe spin out issue triage from PR triage and integrate adding saved replies to development guide, under triage #23109 and add something like the following flowchart based on lots of convos at scipy about best practice:

graph TD;
    A(is the issue) -->unresolvable-->C(explain why <br /> technically infeasible <br /> or not aligned with <br /> values/mission/scope/etc);
    A-->resolved--> B(explain why, <br /> link to docs, API, etc) ;
    A-->resolvable-->D(work w/ issue author <br /> on an implementable <br /> code/docs solution)
   A-->E(not sure)-->F(ask for more information) -->A;

self merged based on approval from @timhoffm #28557 (review)