Adds a new documentation page for Gemini CLI extensions, explaining how
to configure and use them. The main README is updated to include a link
to the new documentation.

This is a follow up after
#329.

## What's Changed
* chore: script to generate examples by @leehagoodjames in
#244
* chore: update readme to state user must copy gemini-dispatch.yml by
@leehagoodjames in
#249
* Relax node requirement and bump actions-utils by @sethvargo in
#253
* feat: support /fix slash command and workflow by @leehagoodjames in
#190
* feat(docs): improve manual setup instructions for workflows by @jerop
in #263
* feat: improve action input descriptions by @jerop in
#264
* fix: enforce COMMENT event type for submit_pending_pull_request_review
by @jerop in
#271
* docs: add fork support documentation for PR review workflow by @jerop
in #268
* feat(action): add support for preview and nightly versions by @jerop
in #281
* feat(workflows): add gemini_cli_version to all workflows by @jerop in
#279
* feat: migrate to new Gemini CLI configuration format by @jerop in
#284
* feat: clean up temp fix for empty API key by @jerop in
#285
* Sort inputs alphabetically in action.yml by @jerop in
#286
* chore: organize workflow inputs alphabetically and add missing
parameters by @jerop in
#288
* feat: add auth input validation by @jerop in
#289
* fix(action): correct quoting in workflow expressions by @jerop in
#293
* fix(validation): improve validation error messages by @jerop in
#297
* fix(validate-inputs): surface errors in logs, annotation, and step
summary by @jerop in
#299
* feat(validate-inputs): downgrade validation failures to warnings by
@jerop in
#300
* fix(validate-inputs): use env-based bash shebang for portability by
@jerop in
#303
* feat: simplify input validation logic by @jerop in
#304
* fix(action): correct input validation script behavior by @jerop in
#309
* feat: add command substitution security warning by @jerop in
#306
* fix(triage): correct duplicate step numbering in workflow by @aladh in
#331
* feat: Optionally install extensions by @CallumHYoung in
#329
* docs(extensions): add documentation for extensions by @jerop in
#340
* chore(workflows): Apply formatting and fix example prompt by @jerop in
#339
* refactor(workflows): improve prompts to use env vars directly by
@jerop in
#342
* fix(review): Remove --source argument to match CLI 8.0 release by
@CallumHYoung in
#347

## New Contributors
* @aladh made their first contribution in
#331
* @CallumHYoung made their first contribution in
#329

**Full Changelog**:
v0.1.12...v0.1.13