Update modus-themes to version 2.1.0

Update modus-themes to version 2.1.0

* doc/misc/modus-themes.org (Customization Options): Document new user
options and relevant changes.
(Option for links, Option for box buttons)
(Option for completion framework aesthetics, Option for mail citations)
(Option for intense markup in Org and others)
(Option for diff buffer looks, Option for org-mode block styles)
(Option for Org agenda constructs)
(Option for the headings' overall style): Update user options.
(More accurate colors in terminal emulators): Add section on how to
improve the accuracy of color reproduction in terminal emulators.
(Case-by-case face specs using the themes' palette)
(Face specs at scale using the themes' palette, Override colors)
(Override colors through blending, Custom Org user faces): Improve
wording.
(Custom Org emphasis faces): Add section about bespoke faces for Org
emphasis markers.
(Backdrop for pdf-tools): Improve sample code.
(Ediff without diff color-coding): Add section about user-level
changes to the style of Ediff buffers.
(Full support for packages or face groups): Update lists of supported
packages.
(Indirectly covered packages): Update list of indirectly supported
packages.
(Note on underlines in compilation buffers): Clarify wording.
(Note on EWW and Elfeed fonts): Be more specific about 'shr' faces.
(Are these color schemes?): Answer frequently asked question.
(Acknowledgements): Update acknowledgements' section.

* etc/themes/modus-themes.el (modus-themes-operandi-colors)
(modus-themes-vivendi-colors): Update color palettes.
(modus-themes-subtle-red, modus-themes-subtle-green)
(modus-themes-subtle-yellow, modus-themes-subtle-blue)
(modus-themes-subtle-magenta, modus-themes-subtle-cyan)
(modus-themes-subtle-neutral, modus-themes-intense-red)
(modus-themes-intense-green, modus-themes-intense-yellow)
(modus-themes-intense-blue, modus-themes-intense-magenta)
(modus-themes-intense-cyan, modus-themes-intense-neutral)
(modus-themes-refine-red, modus-themes-refine-green)
(modus-themes-refine-yellow, modus-themes-refine-blue)
(modus-themes-refine-magenta, modus-themes-refine-cyan)
(modus-themes-nuanced-red, modus-themes-nuanced-green)
(modus-themes-nuanced-yellow, modus-themes-nuanced-blue)
(modus-themes-nuanced-magenta, modus-themes-nuanced-cyan)
(modus-themes-special-cold, modus-themes-special-mild)
(modus-themes-special-warm, modus-themes-special-calm)
(modus-themes-diff-added, modus-themes-diff-changed)
(modus-themes-diff-removed, modus-themes-diff-refine-added)
(modus-themes-diff-refine-changed, modus-themes-diff-refine-removed)
(modus-themes-diff-focus-added, modus-themes-diff-focus-changed)
(modus-themes-diff-focus-removed, modus-themes-diff-heading)
(modus-themes-heading-1, modus-themes-heading-2, modus-themes-heading-3)
(modus-themes-heading-4, modus-themes-heading-5, modus-themes-heading-6)
(modus-themes-heading-7, modus-themes-heading-8, modus-themes-hl-line)
(modus-themes-variable-pitch, modus-themes-lang-note)
(modus-themes-lang-warning, modus-themes-lang-error)
(modus-themes-reset-soft, modus-themes-reset-hard): Fix typo in
':group'.
(modus-themes-markup-code): Add face.
(modus-themes--headings-choice, modus-themes-headings)
(modus-themes-org-agenda): Update documentation about typographic
weights.
(modus-themes-org-blocks, modus-themes-completions): Update doc string
and add new stylistic variant.
(modus-themes-intense-markup): Deprecate old user option.  Replace it
with 'modus-themes-markup'.
(modus-themes-mail-citations): Update doc string and add another
stylistic variant.
(modus-themes-box-buttons): Add new user option.
(modus-themes--slant): Tweak how slant is applied.
(modus-themes--markup, modus-themes--prompt, modus-themes--paren)
(modus-themes--syntax-foreground, modus-themes--syntax-extra)
(modus-themes--syntax-string, modus-themes--syntax-comment)
(modus-themes--heading, modus-themes--agenda-structure)
(modus-themes--agenda-habit, modus-themes--org-block-delim)
(modus-themes--mode-line-attrs, modus-themes--standard-completions)
(modus-themes--link, modus-themes--link-color, modus-themes--region)
(modus-themes--hl-line, modus-themes--mail-cite): Tweak private
functions.
(modus-themes--button): Adjust private function.
(modus-themes--heading-weights): Deprecate it.  Replace by
'modus-themes-weights'.
(modus-themes-faces): Update faces.

* etc/themes/modus-vivendi-theme.el
* etc/themes/modus-operandi-theme.el: Bump version number.

For a detailed change log, read:
<https://protesilaos.com/codelog/2022-02-17-modus-themes-2-1-0/>.