docs: add documentation style guide for Backstage contributors (#33538)

* docs: add documentation style guide for Backstage contributors

Adapted from the Kubernetes documentation style guide, tailored
for the Backstage project with Backstage-specific terminology,
Docusaurus conventions, tone guidelines, and project word list.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: aramissennyeydd <aramis.sennyey@doordash.com>

* docs: soften "we" guidance and link style guide from CONTRIBUTING.md

Relax the "avoid we" rule to allow "we" in tutorials and walkthroughs
where it means "you and I, working through this together." Also add
references to the style guide from CONTRIBUTING.md and AGENTS.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: aramissennyeydd <aramis.sennyey@doordash.com>

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Aramis Sennyey <159921952+aramissennyeydd@users.noreply.github.com>

* fix link checker

Signed-off-by: aramissennyeydd <aramis.sennyey@doordash.com>

* skip block

Signed-off-by: aramissennyeydd <aramis.sennyey@doordash.com>

* address feedback

Signed-off-by: aramissennyeydd <aramis.sennyey@doordash.com>

---------

Signed-off-by: aramissennyeydd <aramis.sennyey@doordash.com>
Signed-off-by: Aramis Sennyey <159921952+aramissennyeydd@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

.github/vale/config/vocabularies/Backstage/accept.txt

@@ -49,6 +49,7 @@ bundler
 bundlers
 bursty
 callout
+callouts
 camelCase
 CDNs
 Chai
@@ -59,6 +60,7 @@ chanwit
 Chanwit
 CI/CD
 classname
+cleye
 cli
 CLIs
 cloudbuild
@@ -138,6 +140,7 @@ dyno
 ecco
 elasticsearch
 Entra
+enums
 env
 Env
 esbuild
@@ -248,6 +251,7 @@ lightbox
 Lightsail
 limitranges
 liveness
+LLMs
 LocalStack
 lockdown
 lockfile
@@ -394,6 +398,7 @@ repos
 requestors
 rerender
 rerenders
+resizable
 resourcequotas
 retryable
 reusability
@@ -562,6 +567,7 @@ VMware
 Vodafone
 VPCs
 VSCode
+walkthroughs
 Wayfair
 Weaveworks
 Webpack
@@ -580,7 +586,3 @@ zod
 Zolotusky
 zoomable
 zsh
-resizable
-enums
-LLMs
-cleye