Eglot: restructure and overhaul documentation

Eglot: restructure and overhaul documentation

* doc/misc/eglot.texi (Starting Eglot): Tweak.
(Using Eglot): Remove link to 'Eglot Variables'.
(Eglot Features): Rework.
(Eglot and Buffers): Rework.
(Eglot Commands): Rework.
(Customizing Eglot): New chapter.
(Customization Variables): Now contains what 'Eglot Variables'
used to contain.
(Other Variables): New section.
(More Customization): New section.
(...elsewhere...): Apply 'fill-paragraph'.

(cherry picked from commit 9e4b626f7eea788966a1ad92d4af76f8b0a2573f)