doc .authinfo; open-page-hook; cache page list; add completion at point; in-browser

[WillForan]

Pushing my and @vincowl's changes your way. I find the package super useful and am excited share back!
Apologies for not providing more atomic commits -- it's a lot of new code written without a lot of expertise. Feel free to reject ruthlessly, change everything without my input, or anything in between.

1. The only change that was on the todo list is maybe dokuwiki-in-browser -- opens the current dokuwiki page in a browser
2. dokuwiki-open-page-hook is run at the end of open-page as a good place to set eg. flyspell-mode or toggling line numbering
3. dokuwiki-cached-page-list holds a list of all pages. list-pages checks for universal argument so e.g. C-u C-c l will refresh the list.
4. with caching menus are a lot faster and completion at point (dokuwiki--capf) is feasible. But setup is a little awkward. dokuwiki-setup sets up the hook for that: (add-hook 'completion-at-point-functions 'dokuwiki--capf nil 'local)
5. dokuwiki-setup also establishes some default keybindings
6. dokuwiki-launch is a convenience function for wrapping setq and open-page
7. dokuwiki-ffap (find "file" at point) provides a way to open a page from a link. (C-c o in default setup)
8. dokuwiki-font-lock-link is incomplete. I wanted to make links clickable (run ffap) but I couldn't figure adding the new pattern

on the documentation readme side, I add

1. more about using .authinfo b/c it was totally new to me (and knowing auth-source-forget-all-cached sooner would have save me a lot of debugging).
2. quelpa-use-package notes -- also new to me. seems like the easiest way to quickly install without managing the repo or waiting for melpa updates
3. screencast! Though it could use a lot of pacing edits

[accidentalrebel]

I appreciate the pull request! I don't use DokuWiki anymore so it'll take a bit of time for me to set it up so I could test these.

[flexibeast]

i've just started using DokuWiki, and will probably be continuing to do so in the near future, so this package is Relevant To My Interests. 🙂

i actually wrote some code to support enabling dokuwiki-mode upon opening a page, before realising i should check if anyone else had done so. 🤪 i've now started running this PR locally, and will test out its changes.

The only thing i've tested so far is whether dokuwiki-mode is enabled on page open, which it does. However, i prefer the approach i took in this regard: it doesn't introduce a dependency on dokuwiki-mode, so users won't be forced to install it even if they're not going to use it.

[flexibeast]

i've just merged this PR to the master branch on my fork, and also modified the dokuwiki--credentials function to support specifying a port (which i need for my own use-case). Will probably continue adding functionality / making changes there as-needed.